ooma

This isn't an a shameless plug or anything, I have no affiliation with these guys other than yes, I did buy a cheapie server from them and it works great! I use it for game servers like L4D & 2142 & all the old classics (red faction, quake series, etc.) The main thing to watch for in my opinion is the Hard Drive you plan on using with it. These are IDE only & the $99 one only accepts 1/4" single-platter drives or maybe a laptop drive. but it's perfect for me :) I popped a 500GB IDE drive in the dual 3.06 Xeon / 2GB Ram one.

dubba dubba dubba dot weirdstuff.com/cgi-bin/category/10174