one of those other things ...
usb <-> ps2 dongles (2 at least)
usb serial port cable (both a 6 foot and a 16 inch one)
a Canadian tire magnetic multi slot screwdriver and a box with - 6" hex drive extender and 72 bits for the damn thing. Covers just about any screw head I've ever seen. (including those damned triangle ones that every oriental aftermarket tat assembler thinks are secure) The bits are from PrincessAuto.
1/4 and 1/2 socket drivers with 4" 6" 14" and 24" extenders, 48 sockets < that lot tend to stay in the spare wheel well of the car >
mini electric screwgun (with charger) < again, in the trunk most often >
Cable end crimping gun and I try to keep a dozen or so ends.
I actually have a parallel port cable, a serial to serial cable, and two USB2 A->B cables.
and not one but 2 spare 64Gb USB thumbdrives.
an 8' tested good cat6 cable. <helps if your TESTER is pooched. And I've met at least one>
I personally carry a 64Gb USB key that has ISO images for a *baseline* recovery image for *all* the OS's I support. And spare CD and DVD disks.