Ebay. There's no real market for old computers except there.

Considering that P3 machines (box, mobo, ram, disk) are listed in ads for $100, though, I'd almost say it's not worth trying to shuck anything older than 5 years. Laptops get a bit more respect. Not much more. New laptops are like $700 from Dell, after all.

If you're near a university, maybe you can arrange for a student club to take that stuff in. My school's computers club was trying to build a cluster of Pentium 2s a while back.

Edit: I'd keep a single Pentium laptop around though, since those remain useful if you're willing to learn Linux to build routers and firewalls.