I think I pack less lightly than some of the folks below... but I pack it in such a way that the car is completely uncluttered. I drive a wagon and fit everything I might need should I break down someplace and have to do an emergency repair. Most of this kit I fit into the back storage hatch:

1) stock jack and toolkit
2) 3 each philips and flat blade screwdrivers (different sizes)
3) channel locks, vice grips
4) magnetic telescoping pick-up tool
5) multimeter
6) set of brake pads and mounting hardware
7) distrib. cap and rotor
8) med gague copper wire, wire cutter/stripper, needle-nosed pliers
9) a handfull of extra lug nuts
10) spare set of spark plugs
11) 1/4 drive socket set
12) welding rods (for fixing dangling exhaust)
13) spare flame trap screen
14) wiper blades
15) pry bar
16) a set of metric wrenches size 8 thru 19, plus a large size for the oil drain plug... and a few small sized SAE wrenches for the odd English nut (no offense to our friends from the UK) on the car
17) timing belt and v-belt set
18) a mini spray can of WD40
19) work gloves... and an odd assortment of zip ties, wire brushes, electrical tape, etc.
20) rags packed in to keep everything from moving around
21) emergency blanket

Overkill you say? Probably. But since it all fits in that little hatch it doesn't get in my way, -- so I figure why not carry it just in case?

I also carry two small, high-intensity LED flashlights (one is a camping headlamp for hands-free lighting), and a Swiss army knife in the glove box. At least two quarts of oil in the storage compartment across from the spare tire. Jumper cables. And finally a small canvas bag with a full set of socket wrenches in all three sizes that I can easily carry with me or stash under the seat if need be.
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'92 245 5-speed, '92 944 GL auto