Two sets of simple to follow instructions to turn a Raspberry PI into a web server from Instructables and WikiHow. The total power consumption for a Raspberry Pi over the course of a year is roughly £3.50 (400 milliamps per hour), so it’s ecologically friendly too.
Other uses for a Raspberry Pi include:
A Minecraft server.
A BitTorrent server.
A web cam server.
PingBin has some more outlandish ideas too.
Or, you could even send your Raspberry Pi into space.