A while ago I was wanted to colocate one of my servers, but I found out that it was pretty expensive if the sever is not rack mounted. I then looked for a relatively inexpensive rack mounted case for this server, but I couldn't find one.
I ended up using a Linux virtual server through TekTonic since it allows me to run Fedora Core, and I can run what ever software I want to run. But there are some limitiations, like I can not get as much performace as I could if I has my own server.
On /. there is a post of someone that created their own 2U rack mounted server. This is the type of computer that I would want to buy or build.