The Raspberry Pi is one of the smallest, most compact, cheap computers I have ever seen and it is super fun to program for! I built this networked counter for a client that wanted to have a large button that incremented a counter that displayed on a big LCD in their store and on their website. I had a blast prototyping this out and doing the python programming. We added audio, a local database and all the network code to update their website via their wifi network.
Recently, I acquired an updated model which I plan to put in an old radio cabinet I have and program it to play virtual radio stations. These might also make decent home servers especially if you want to monitor hardware sensors.
My only hesitation to buy more of these is that already you can pick up an old smart phone for about the same price as the pi + accessories, which will have all the capabilities in a smaller enclosure WITH a screen, i.e. my iPhone 3G that is collecting dust.