• Tools - most everyone has already covered everything here, but I'll add long-handled small Phillips screwdrivers
• Soldering iron
• Electrical tape, zip ties, Velcro strips
• Cat5/6 crimper, RJ45 ends, and cable, assorted pre-made patch cables
• Universal laptop charger, and I also carry a Dell charger
• USB DVD Drive
• 2.5 IDE/3.5 IDE/SATA to USB adapter
• Spare 2.5" and 3.5" SATA HDD (In the middle of nowhere, can't just run to the store and buy one)
• USB wireless adapter
• USB cables - A-B, Mini, Micro
• Other various cables - HDMI (standard,mini,micro), VGA, DVI, power, parallel, SATA, IDE
• Various spare screws and standoffs
• Windows install discs (XP SP3, Vista, 7, 8 - 32 and 64 bit versions as appropriate)
• Ubuntu live boot discs
• 500GB external USB drive for data backup

And most importantly: Read only USB drive for cleaning infected computers. You don't want your own drive getting infected. I use a Kanguru.

Edit: Torx screwdrivers and a USB mouse are handy too