Recently I have been playing with several JeeNodes microcontrollers from JeeLabs. The JeeNode is an Arduino clone with a built in wireless transceiver for data transmission.
Although not as fully featured, these transceivers are much cheaper than the similar Xbee units, and so can allow for lower cost experiments in radio communication networks. After some initial experiments with JeeLabs RF12 Arduino library I ended up writing my own library. I am currently working on some possible applications.
You can find the source code for the Radio library on GitHub here.