I've had several collections of 'throwaway' computers, with my last 486 and P100 going to that big recycling heap in the sky a few years back.
Right now, I've got nothing super old, but I've been keeping an AMD K6-II alive through the ages. It was my first IBM-compatible machine (after a C= plus/4) and I still use it regularly as a development box (Debian Stable in console mode all around. vi, gcc, perl, ssh, ftp, lynx. What more do you need?).
It is fortunate to have 2(!) USB ports on an add-in interface, so I can still plug a MS Ergo 4000 keyboard and modern optical mouse. Most of the hardware is original and all works, but the three things I've had to replace periodically are the optical drives (several), cpu fan (twice) and memory (twice). Up until about 5 years ago, compatible parts were plentiful from old computers, but I haven't seen the right sized fan or any SDRAM for the picking in ages, and it's now getting harder and harder to find IDE anything, even used.
I still enjoy the hell out of my i5 (and other smatterings of computers lying about) but I'll be sad when I have to put the ol K6 down.