I have an old Dell Optiplex 755 SFF (Small Form Factor) that is one of my favorite PCs.
You can get one like it here, for about $180... http://cgi.ebay.com?ViewItem&item=300644713361. Mine has a 300GB hard drive and 4GB of RAM (All easily and cheaply upgradable).
On Saturday (Dec 31, 2011) I was attempting to install Ubuntu 11.10 on it, but I couldn't boot to the install DVD. I had downloaded the correct Ubuntu image, and burned it to DVD on my Intel iMac (OSX Lion). After the burn process, it verified the disk and everything appeared ok. The problem was, it wasn't bootable. After wasting a few hours trying to make the machine boot from a "bootable" DVD, I gave up, threw the DVD away and re-downloaded the iso (just in c... [More]