Hey, just ran across this site. They are giving away free PCs if you agree to watch 3 min of adverts. every hour. but (I think) if used as a server, you could probably leave it on and not even notice that the ads play. Oh well, I put my name in..
http://www.metronomy.com/
EDIT: pulled a classic newb
and didn't read all the terms. I'm not UK resident. :(