Personally, I've never thrown away a Mac. I've always had someone to give or sell it to. I suppose it's a testament to the longevity of each unit.
The first Mac I owned, a Performa 6400, went to a friend of mine in Albuquerque. It had a good life: I installed a G3 upgrade card in its personality slot when its 603e processor wasn't cutting it anymore. I don't reckon she's using it still, though.
After that, I bought a Power Mac G4 with a 17-inch Studio Display (in matching graphite, of course). My aunt and uncle are still using that computer, and it's running just fine for the basic email and web browsing they need. It's probably more than they'll ever need, come to think of it.
Then came my iMac G5, which I sold to my previous employer for $500 or so. They sort of ... uh ... downsized after I left, so I'm not sure if it's being used right now. It runs great, though. I installed a new power supply in it a month or two before I sold it.
My MacBook and Intel iMac are running great for me right now, though, and I have no idea how they'll spend their retirement after they've done their bit for king and country.
Where have your old Macs gone? Schools? Parents? Kids? Share your misty-eyed memories of Macs past in comments.