when I was at university, we often raided skips outside the computing center, and found a few machine that way.
(entirely FOC obviously)
A few years ago the company that I work for moved offices, lots of the accumulated junk was going for free, I picked up an old (tower based) dell power edge server. for free... and a free laptop...
Mostly now though I find that I seem to ascribe value to PCs when they just don't have it.
I have my main machine (laptop) and I've got a machine running hyper-v with a few VMs on it for a test environment.
after that a couple of raspberry pis is all I need.
I still have really old machines just sitting waiting to be powered back on in case I need to recover data though, (not that I'd remember the password even if I did turn them back on!)