I pack around a 1GB Sandisk with Windows 2000 & XP SPs, updates and various little utilities.
Bend nose pliers, a couple of different screwdrivers, a small Rayovac flashlight (plastic case model). The plastic housing makes it much easier to hold in my teeth so my hands are free.
I also carry some 91% Isopropyl, roll of toilet paper, a can of 3-In-One oil, a roll of foil duct tape. A few spare fans of different sizes (40mm, 80's & 92's).
A spare ATX PS and some thermal compound.
Spare screws and some small zip-ties.