Low-Powered Personal Servers?

antifoidulus asks: "Being the proud owner of a PowerBook, I have but one complaint when it comes to my computing experience: the lack of an 'always-on' web/database server that would allow me to work on some personal programming projects, since I don't like having my PowerBook on 24/7. I could just buy an Intel box, but looking at some of the horror stories of how much power P4s consume, and living in Germany where electricity is not cheap, I wanted to see what suggestions the Slashdot community has for low-cost, low-power, headless servers. My only requirements are that it can run Linux and preferably cost less than $500. Is this possible? What architecture should I go with?"