I've put together the beginnings of one for myself.
-6pc Stanley precision screwdriver set from a hardware store
-250mb flash drive
-flexible USB LED light (less useful now that I've sold my laptop)
-adapters for USB-PS2 and PS2-USB
-burned CD-Rs with useful software, including several bootable ones
-spare telephone cord and LAN cable
Carrying case: Repurposed from CaseLogic. It was designed to carry a portable DVD player and several discs.
Belkin Tool kit (I added in tie wraps, electrical tape, and an anti-static wrist strap)
CD case (generic) with lots of common stuff (LiveCD's, partition software, low level disk software, etc)
512mb thumb drive with security patches, every network driver known to man, etc
Short of needing actual parts, that has always gotten me by.
I think what I'll do is make my own tool kit...however there are some stuff i'd like to get for it but don't know where i can buy singles. Currently at work we use these: http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=171941&CatId=1801 . Our kits are rather old and the screwdrivers are crap and don't work like there suppose to anymore. Also, our allen wrenches from our belkin sets are worn out. Plus, our mini screwdrivers are gone!? Where can i get the precious 2-way mini screwdriver?
Making your own kit is the way to go. Some of the tools, especially the screwdrivers that come in those kits use cheap metal and warp if you apply force to remove a stubborn screw.
Gotta agree those Wiha screwdrivers are worth the money. According to their site, they have sets that are ESD safe and interchangable bits
My kit:
Stanley phillips screwdriver
Mag Flashlight
Precision Screwdriver kit from Sears
Leatherman Tool
I'd add a Stanley knife to that list, as well. The proper one, not those snap-off-blade craft knife things. Throw in a 5mm socket as well, or something to tighten down motherboard standoffs.
As a knife, it works well for opening packaging and keeping away fiddle-fingered people.
As a bonus, you can disassemble it for a good, sharp edge to clean off surfaces (say, CPU IHSs, before the alcohol) and replace the blades.