An Arduino-based differential solar controller for a domestic solar hot water system. In three parts: hardware with custom PCB shield, Arduino sketch / firmware, and a set of scripts to monitor, record, graph, and display system output using a Google Gadget.
My very first open-source project on Sourceforge.net a long, long time ago. x10mp3 is a plug-in to control XMMS using an X10 Mp3Anywhere remote, which later expanded to include a plugin for WinAmp. Horribly out of date, it is still being downloaded today.
Build a wireless humidity sensor and monitor the output to get notified via Twitter when the kettle boils. A good introduction to XBee radios. Also includes Java and Arduino libraries for the HIH4030 and HIH4031 humidity sensor.