I currently have a model B (the old one with 256MB) at home with an uptime of almost 500 days. It is used to share an external drive to the network, serves some static pages to the web and is a gameserver (nethack, moria, etc) for me and a couple of friends. It is also my ssh-entrypoint from the outside, and running a mumble-server (umurmur) which can handle a couple of people without problems.

I've been meaning to do something cool with it, but the limited power is, well, limitng. But it excels everywhere where you just need any computer.