I've got a couple of them that I am playing with.
You can get expansion boards to quickly add peripherals to prototype with. The one I use is a Gertboard. You can get dongle for WiFi and Bluetooth so you can "talk" to it using a phone if you want to. So, for example, you could use your phone to get it to activate any relays which you choose to connect it to. There are some fairly whacky ideas out there.
One idea I am working on is hooking it up to an OBD port and getting it to translate some of the diagnostic codes into audible warnings or alarms, or to work like a diagnostic trip computer, eg recording average and maximum coolant temperature, etc. It has a "general Purpose I/O Bus", so using A/D and D/A converters and other gizmos, you can basically connect if to anything that can be measured/ monitored or controlled electronically.
