I bought an Android set-top-box, the Flexiview FV-1, ($120ish, 1Ghz Cortex A8) at the end of last year and got Debian running on it.

http://projectgus.com/tag/fv-1/

I haven't run one 24/7 for more than a week or two though, so I don't really know how good it would be as a server.