Here are some of the things I keep in my computer toolkit:
#1 Phillips screw driver
#2 Phillips screw driver
Flat blade screw driver
Small flat blade screw driver
Long straight needle nose pliers
Long curved tip needle nose pliers
Flush cutting wire cutters
1/4†nut driver
5mm nut driver
Small flashlight
Machinist's scribe
Sheet metal nibbler
Small, med, larger tie wraps
Small mirror with extendable handle
Small metal ruler
Wire stripper
Heat shrink, various sizes
Extra AC power cord
Power supply tester
Heat sink compound
Exacto razor with handle
A few anti-static plastic bags
Mechanical AC power switch (almost never used these days)
Ethernet end crimper
RJ45 ends
Extra SATA and PATA cables
Chip puller (Probably don't need these days)
Small crescent wrench
Screws, fan mounting
Screws, case - course
Screws, case – fine
Motherboard stand offs
Power adapters, 4 pin molex to, Y adapters, 4pin fan, etc.
Pen and paper
Sharpie
Dial Caliper