If you're in the market for a good used server, you should check in with the Wikimedia Foundation that runs Wikipedia. They've decided to upgrade, and they're giving away their old hardware.
The servers are about three years old, and typically have 2.5GHz dual core processors with between three and 32Gb of RAM, and hard drives from 80-320Gb. Not exactly state of the art, but hey, they're free.
There are however a couple of catches. You must be based in the USA, and you must be either a registered non profit organization, or someone who actually does work for Wikipedia. So if you were thinking this would be a cheap way to get your online porn business up and running, forget it. The request must include some details about how you actually plan to use the server, so you'd better come up with something pretty good if you want to make the cut.
The servers are being shipped on a first come first served basis, and it looks like they even throw in the shipping.