Spare computer parts for everything tested to be in perfect working condition. You said you had monitors, which is good. Here's some more ideas:
Video cards - Some PCI Express, and an AGP and PCI.
RAM - Both DDR, DDR2, and maybe DDR3 and PC133 SDRAM.
Power supplies - Just a few that are tested to be working properly, with good voltages. Which reminds me, get a multimeter to test PSUs. Here's a guide:
Motherboards and processors - Not really a necessity, but may be nice to have. Get at least LGA 775 and AM2, 939 and 478 would be good as well.
Pretty much all the diagnostic software you'll ever need, compiled into 1 CD. Everything from hard drive diagnostic tools to memtest to stress tests.