you can possibly find:
dc motors out of cdrom/dvd/floppy disk drives
as well as 4 pin stepper motors that can be used with a microcontrolller
nylon gears of various sizes
LEDs which can be used for half duplex communications or for show
rare earth magnets from the hard drives
easily workable metal from the drive enclosures, tin snips ftw!
various connectors that can be desoldered for later use
heat sinks, great when your pushing something hard
the plastic pieces can be cut and used for various applications with a little imagination and sometimes heat lol
the power supply for a benchtop power supply, check instructables.com for howtos (you can also just use a AC to DC wall wart)
fans for cooling or whatever
basically everything can be highly effective for reuse but the motherboard, cpu and addon cards and im sure theres exceptions to even that.