I doubt I’ll end up following through, but I’ve been kicking around the idea of getting a cheap used server and shipping it off for colo. eBay has some pretty good deals, below. Pay attention to shipping, though, as servers can be extremely heavy.

• A mere $299 buys this one, a 1U Proliant with dual 3.2 GHz Xeons, 4GB RAM, and dual 73 GB SCSI disks at 15K RPM. For my purposes I want something with gobs of disk space, but if you’re looking for something fast, this might be a good pick.
• A lot of 10 Dual Opteron Rackable 1Us, $1300. The Rackables are 1U boxes that are half-depth, so that you can load 42 in the front and 42 in the back of a standard 42U cabinet. (Might want to think about where all the heat goes before you do that…) Each of these has dual Opteron 246 processors (2 GHz), 4 GB RAM, and a 120 GB hard drive. Opterons don’t seem to be the most popular chips, but at $130 each (if you buy 10…), these are pretty affordable. And from my unusual perspective, it’s a bonus that it’s got a standard ATA disk: it’s easy to plop a 500GB one in. Not sure if there’s a lot of room for many drives, though.
• $375 with shipping buys this Proliant, which is 2U with dual 3 GHz Xeons, 6 GB RAM, and 6x 73 GB disks. Plus dual power supplies and onboard RAID. (Though the latter is almost standard on servers like this.)

If I knew people who wanted to take about eight servers off my hands (at cost, plus shipping), I’d be tempted to pick up the Rackables. The thing would be more than capable for a decent colo (unless you need RAID or a chip that supports hardware virtualization…), and would make an insanely awesome home server, whether it’s a network fileserver (again, unless you count on RAID, which you should for a home fileserver), or a ridiculously-overpowered firewall. (Maybe throw pfSense on it, in which case you can do fancy stuff and even allow VPN access to your home network?)