Don't forget the Winternals stuff.
Particularly the administrators pack. You can "load" a non-booting machines registry, for editing, etc. Not to mention the other tools included....
Also, get yourself a decent post card. These can save a lot of time.
A silly gadget, but worth mentioning - considering the regularly failing psu's these days, you can get a very cheap little device, about the size of a box of matches, with rows of LED's for each voltage line, that you simply plug the psu into (atx plug, molex's, etc) Instant psu diagnosis
(no more stuffing probes into each pin!)
Just some quick thoughts