I have four RPi boards. One monitors my UPSs, cleanly suspending my server when the power goes out and sending wake-on-LAN massages to it when the power comes back up so that the UPS only needs to drive my switch and AP, one has a camera board and does motion detection to spot people coming into my office, one is currently operating as a Bluetooth LE beacon for testing the new iOS iBeacon stuff and one is just for tinkering. Most of these have a few other services running on them too (two have I2C thermometers on them).
I see a lot of negative comments about the Pi being underpowered. Perhaps if what you want to do is run FPS games or you are trying to run Big Data analytics then this is true but it's plenty powerful enough for a whole host of service tasks. It's not that many years ago that the Pi's level of power would have been considered a high-end desktop configuration. The purpose of the device is to give kids a low-cost entry into programming and it does just that. On top, at $25 for a Model A its fine to put in 'dangerous' places where something bad might happen to it (like outdoors, driving the sensors and servos for my Halloween decorations). No, I don't have my MongoDB server on a Raspberry Pi, but for many many projects they are just about perfect.