If you live near Portland, OR, you could always try Free Geek - they often have usable stuff for next-to-nothing, and the proceeds go to charity. From the looks of it, they even ship (though if you live outside of Oregon, you might want to ask how much they'd charge for shipping.)

(Lost count of how much fairly high-horsepower-but-out-of-support-contract hardware we'd dropped off there from nearly everyone I worked for in/around PDX...)