An XBEE / MQTT gateway, written in Node.js.  XBee API frames received by the gateway are published to MQTT, and MQTT messages subscribed to by the gateway are transmitted over the XBee network.    
ThingSpeak Java Client
Update channels and parse feeds in Java with this client library for the ThingSpeak API.
DSC - Differential Solar Controller
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 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  Horribly out of date, it is still being downloaded today.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.
Java access to the gphoto2 library using JNA.  Like gphoto2-java, but all wrapped up in a simpler, documented API.