Inveno's ICT_Works blog recently advertised their awesome addition to the ICT4D world - a solid toolkit to carry to the field.
I made a much lower-tech personal version many years ago, as an IT Peace Corps volunteer. Mine of course was (a) very low cost and (b) designed to travel stuck in side-pocket of a backpack on a crowded country-bound bus. OK, it was really just a few screwdrivers, a 3-2 prong electrical adapter, and a flashlight.
I also always carried around:
1 ~3' crossover cable or crossover adapter (no faster way to test networking!)
1 multi-head cable tool (the kind with a core USB retractable cable and a pouch of other cable-heads to turn it into a phone, network, mini/micro/device USB, firewire, etc. -- easy way to carry around "the right cable for the job" when you're not sure what today's job might be)
My favorite toolkit items were more around the software end of the spectrum, though:
1 bootable USB stick with DamnSmallLinux and a PortableApps Suite
1 bootable floppy with WinXP/NT admin password reset tool
1 BartPE bootable XP CD with anti-virus and diagnostic tools
1 Knoppix or other Linux LiveCD that will work on a wide range of hardware and let you extract files from the HDD
1 CD and USB stick of common free/shareware/OSS software tools - anti-virus, various anti-mal/spy-ware, registry cleaners, zip/archive software, OpenOffice, PDF creation tools, and so on.