I built a media server that was actually approved by my wife for placement in the living room. I started with some furniture from Target and put a computer in it. I say media server instead of media center because it's only for music and pictures. No PVR or fancy stuff on a 500 MHz AMD.
Check it out at my blog.
edit: Combined multiple threads from Worklogs, Galleries and the Chatterbox into this single thread, as they all had basically identical original posts.