If the computer as a whole still functions, I keep it and find a use for it. For example, I still have my Celeron 366 which I installed xubuntu on and run as a p2p downloading machine. It's kinda geeky cool to be able to draw functionality and "value" out of a 12 year old machine with a modern OS. If I wanted to dump my Pentium 4 machine, I COULD give it to my sister's work place. Their machines are from the same era and work just fine for their uses.
As for parts, I keep some old working parts as backup, like a couple of 120GB IDE HDs. Otherwise, I give them to a recycling center. I suppose I COULD sell my working parts, but I'm not set up to sell anything online, and I feel it would just be too much of a hassle to have to wait for buyers and then pack the parts up and ship them.
I have a USB zip drive with a couple of 100MB zip disks in my collection, and ironically, I can't figure out what to do with them. USB flash drives and gigabit LANs have made them pretty useless to me.