DIY: What are the tools that every Do-it-Yourselfer must own?
What are the tools that every DIY'er should have? This is a community wiki as there is no one right answer.
Instructions:
- One item per answer (so they can be voted on - answers with multiple items will be voted down)
- Clear formatting like this:
[ITEM NAME]
[Reason why item is essential/useful/good]
[Photo / other info]
A cordless drill
No doubt about it. I bought a DeWalt 14.4V three years ago, and it's been invaluable to me. alt text http://www.homedepot.com/catalog/productImages/300/34/34c4b1f5-f4c7-465c-b9bb-f0a9ba2cd2ef_300.jpg
A good weight crowbar
Use it for lifting, prying, removing, bashing, demolishing and most importantly, against zombies and headcrabs.
A good, stiff measuring tape
Utility Knife
For about $10-15 USD you can get 100 utility knife blades, so you don't have to worry about sharpening your knife/breaking the blade (except your eyes of course!).
I use mine for all sorts of stuff.
A first aid kit
...that is easy to find!
A multimeter

(Image licensed under the Creative Commons)
Needle-nose pliers

These are the most often used tool in my toolbox, not that they ever make it back into the toolbox.
Hammers and screwdrivers.
You aren't going to get much done without them...
A good jigsaw can be very helpful for many tasks as well.
Speed square
Locking pliers
Commonly known by the genericised trademark "Vise-Grip"

I find that I use it one way or another on every project I do.
A power miter saw (compound if possible).
It will cut anything you'd cut with a circular saw (other than large sheets), and you'll also be able to cut any trim pieces you'll ever need. Adding and replacing trim is a relatively easy thing to do and can quickly add value and better the appearance of a house. Few things come as close the a bang-for-the-buck arena.
alt text http://www.besthometoolsale.com/images/dewalt-miter-saw-dw717.jpg
Non-Contact Voltage Tester.
This comes in very handy to make sure you turned off the correct circuit breaker before doing any electrical work. And really helps if you have some funky wiring in your house and not everything in a single box is on the same circuit.
Non-Contact Voltage Tester http://ecx.images-amazon.com/images/I/2149063ZRQL._SL500_AA300_.jpg
Screwdriver set
screwdrivers http://img339.imageshack.us/img339/7803/31032044.jpg
Your exact types will differ by country, but you likely want:
- Slot-head (2 or 3 sizes)
- Phillips (atleast size #3, maybe #2 and #4)
- Robertson (atleast red, green, and black)
Rubber handles will save your hands after a bit of use. Also, try to find black tipped drivers, as this means they're hardened and shouldn't wear down as quickly.
A reciprocating saw (a.ka. sawzall) -
- Makes short work of any tearout job.
- Gets into places that other saws can't.
- Great stand-in for a chainsaw outside (for small stuff)
Set of standard and metric allen keys.
alt text http://ecx.images-amazon.com/images/I/51DD9ETKNZL._SL500_AA300_.jpg
Quick clamps.
alt text http://www.diyskate.com/img/ramps/tools/quick_clamp.jpg
about a million times better than these:

A circular saw of course.
And don't skimp - you'll use it enough that it's worthwhile to spend the money and get a decent (and light) one.
Good levels of different sizes
A good set of channel lock pliers (multiple sizes).
Use them on almost every job, especially plumbing.
alt text http://img294.imageshack.us/img294/6085/39540403.jpg
5-in-1 Tool http://ecx.images-amazon.com/images/I/31p1ZesJSlL._SL500_AA300_.jpg
Great for scraping, pealing, poking. I use it all the time (and it is stronger than a putty knife).
A dremel with various bits: cut-off wheel, stone, sandpaper, polisher, etc.
Fire extingusher
http://a.imageshack.us/img291/47/firekz.jpg
I have something similar to this Kiddie Single-use from Home Depot because of its ABC rating:
Suitable for use on Class A (trash, wood, & paper), Class B (liquids & gases) and Class C fires (energized electrical equipment). The Full Home unit is fitted with a pressure gauge that provides at-a-glance status, is manufactured from lightwieght aluminum and a tough nylon valve assembly.
- Mult-Purpose Dry Chemical
- UL Listed / Rated 1-A, 10-B:C
- Suitable for use on most common fires
Better to have it and not need it!
Complete socket wrench set for 1/4", 3/8", and 1/2" drives and box/closed end wrenches. Should include shallow and deep sockets. Places like Sears will carry an affordable portable fitted toolbox with all the sockets and wrenches.
Rubber mallet - good for things where a hammer will just dent things - also good for adjusting things with taps.
I'm surprised no one has mentioned them but I think a good set of chisels are invaluable. Any time you're working with wood, a sharp set of chisels can be the key to getting a good fit and finish.
A Block Plane
Fits in a toolbelt or toolbox. Comes out every time something almost fits. Saves eight million trips back to the table saw. Handles simple rounding and shaping.

Safety Glasses

Not really a "tool" per se, but one thing I ALWAYS and is stress ALWAYS make sure I have in my tool box and wear all the time.
Had a friend that got metal in his eye and had to have the metal drilled out, creeped me out so much that I now wear safety glasses anytime I do any kind of work around the house.
A small pancake compressor. You can easily borrow / rent the tools, but having the compressor for so many jobs comes in handy. Can also be used without a tool to blow stuff off, fill tires, etc.
A Good Multi-tool

My personal favourite tool was my Gerber - though I know some people also like/prefer Leathermans. This is one thing where going cheap is not worth it. Anything Gerber, -Leatherman, -SOG, or any other quality tool brand really shines when compared with a cheap $10 variety. Seriously - save up for a few months and buy a good one. You'll be very glad you did.
I used mine almost daily until I lost it :'(
I'm saving up for a new one :D
A ratchet with a gator-grip head. That is, unless unlike myself, you really like digging through a toolbox full of sockets for every job and can't identify the sizes on sight.
BTW: I have no interest in the company that makes Gator-grip, I just didn't know the generic name for this type of tool (if there is one).
alt text http://img651.imageshack.us/img651/4903/gatorgripwrench.jpg
A hack saw.
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A Good Flashlight
Regular and Small versions are good to have - I'm constantly looking for one when trying to fix something.
A decent table saw
After getting a cordless drill and a power miter saw.

I love Japanese saws. They cut on the pull stroke and leaves a very narrow kerf (the channel that it cuts). I find that it cuts through wood a lot easier. Cutting on the pull stroke means that the blade won't bend. For saws that cut on the push stroke (most western saws), the blade might bend as you push.
I have this all purpose one, Japanese Modern Ryoba Saw, at home. One side is for ripping (cutting to a certain width) the other is for cross cutting (cutting to a certain length)

A straight edge is really important. I actually have a 48' long aluminum ruler/straight edge that has a significant depth. It's great for drawing long lines and I sometimes use it as a guide for a circular saw or router.
My outlet tester; beats sticking a paperclip in the outlet to see if it's live...
alt text http://www.acmehowto.com/howto/homemaintenance/electrical/images/no/outlettester.jpg
A collection of good-quality clamps of different sizes, including some that can be tightened with one hand.
Adjustable Spanner (Wrench)

Can be used on a wide variety of nut sizes.
Gloves. A good pair will save you several trips to the first aid kit.

Workbench with a Vice
This may not count as a "tool", but not having one can make DIY projects difficult. I don't have one, and trying to sharpen a mower blade on your kitchen table is sub-optimal. Then there was the metal bracket I needed to drill a hole into... holding it with my hand...
I've had my eye on this bench from Home Depot. I like the steel work surface. I will need to drill holes to bolt the vice to the top, though.
Update: Last weekend, I bought this bench along with a 6" vice. My dad and I assembled it as a weekend project. We drilled holes for the vice and secured it with 1/2" bolts. Overall I am pleased with it. It did require a drill for assembly, which was a little unexpected. Here are the photos. (Sorry for the poor lighting, a good workshop light is next on my list!)
alt text http://www.lcdpayless.com/images/ryobi/RyobiOnePlusImpactDriver1.jpg
The ultimate compliment to your cordless drill - it does everything my cordless drill sucked at doing!
- Drives in screws without mangling the fastener head (phillips head screws anyone?)
- Removes rusted/stuck items like you wouldn't believe (Read: no rounded out screws or rounded off nuts/bolts)
- Cordless Drill will not drive a 3" screw into a stud - Impact Driver doesn't break a sweat.
- Compared with cordless drill, much less torque transferred to your wrist makes it more comfortable to operate
Lineman's Pliers
Lineman's pliers are a sort of heavy duty combination of wire-cutter and pliers. A pair of lineman's pliers accompanied by a pair of wire strippers is about all you need to do just about any home wiring project. I've also used it to pull out staples and nails, bend sheetmetal, cut open tubes of silicon, and even as a light-duty makeshift hammer when I didn't want to walk all the way back to the garage.

Something you'll definitely want if you're doing any kind of drilling into masonry or stone. A bonus is that these can typically be used as just a regular drill by turning off the hammering action, so it's basically a 2-in-1 tool. Some even let you turn off the drilling action so you can use it like a small jackhammer.
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Head lamp
Much easier than having someone else hold a flashlight, or trying to balance the light against the tool box.

You'll definitely need a torpedo level. I don't have a lot of storage space, so I just have one of these torpedo levels.
A portable workbench: I use my Workmate all the time:
Watchmakers Screwdriver Set
It seems like more and more things around the home use those pesky little screws. Also don't forget all those kids toys, electronics, etc that like to use those same little pesky screws.
Oscillating tool
Those cheapo harbor freight version of the Fein Multi-master et al. are nearly as good but much lighter on the pocketbook:
A decent quarter-sheet, general purpose palm sander. Especially one with decent fastening capabilities.
Bosch 1297 http://www.plumbersurplus.com/images/prod/5/Bosch-1297DK-rw-80730-148351.jpg
Am surprised that no one has mentioned Duck Tape (Duct tape?) and 3 in 1 oil.... :)
Aviation shears (tin snips)

These are like giant wire cutters. They are designed to give you a massive amount of leverage from a squeeze of the hand. They're great for cutting anything that will fit in the jaws : wire, nails, sheet metal, conduit, doweling, and corner beads.
A good knife.
Always in my pocket when working on something. (Knives like the one pictured seem impossible to find in hardware stores in the U.S. Where are they sold?)

(Medium to Heavy Duty) Extension Lead/Cord
Leads on corded power-tools aren't always long enough to reach from power-outlet to working area.
Snake Light
alt text http://ecx.images-amazon.com/images/I/41n6iyi-55L._SL500_AA300_.jpg
Stop holding a flashlight in your mouth.
A good solid 3-4 pound drilling hammer
...because regular hammers can chip when struck against hardened steel (i.e. chisels, star drills, hardened nails, etc.).

A Picquic.

[I would have left this as a comment on the screwdriver-set answer, but I don't have sufficient rep.]
If you're in Canada, the one tool I'd recommend above any other is the standard Picquic combination screwdriver. Canadian Tire's page is probably the best example -- judging by the reviews, I'm not the only one who loves it. :)
I've used mine for years, on almost every job I've done around the house. In a pinch, the bits also fit well into a cordless drill. There are probably good equivalents in other countries, though I've not run across any myself.
A slide gauge

If you want to know what size bolt you're supposed to use, how deep a hole you need to drill or whenever precise measurement is needed.
A laser level (a cheap one will do just fine).
alt text http://img7.imageshack.us/img7/5220/straitlinelaserlevel150.jpg
(A high quality) Combination Square

Transferring measurements, marking out 90° & 45° angles, scribing parallel lines...
Chalk Line

Snapping, setting out straight lines.
(Sharp) Pencils
You need something to layout, set-out, all those cool DIY projects you are going to do.
Don't forget a pencil sharper, if you can't get a good sharp point via a knife.
Shrinking tube and hot glue - use them all the time.
The combination of shrinking tube and hot glue makes smooth, sealed and water-proof solids of any basic shape. Ideal to create custom-shaped plugs or jacks.
Toolbucket. Love mine. It's amazing what all you can get into one of these.
A good Do-it-yourself book.

Cordless 18v lithium ion impact driver. Drives screws and bolts so smoothly, quickly, and without torquing your wrist. Also relatively light, small, hangs on a belt, and battery lasts a long time.
Makita BTD144 is one good example.
(A good quality) Handsaw
Don't underestimate the usefulness of this old school hand-tool.
Cordless (circular) trim saw. Fits into tighter quarters, better for making short cuts than the full-size circular saw.
Toolbox
You need somewhere to store, organise and keep safe all those essential DIY tools.
Plumb-bob
Sure you can use a Spirit Level to check if something is plumb, but there are occasions when a Plumb-Bob just makes more sense...
Nail Pincers & Nail Bars

Makes pulling out nails a breeze.
Palm Router
Cheap enough for most DIYers, you can pick up a good quality one for around $100.
Has enough power (normally about 1HP) for most DIY jobs.
Can be used to make a variety of DIY projects around the home that much easier.
A Diamond Sharpening Stone
For honing and keeping those plane blades, wood chisels, knives, etc good n' sharp.
Stud finder.
In case you need to hang something that requires more support than a drywall has.
The Basic tools needed in your tool box to be a Do It Yourselfer!!
