When working on the HCD project it came apparent that the small radio chips used could be used for general purpose wireless communication.
The BK2421 based radio turns out to be compatible with a lot of other devices like the RFM70 or the nRF24L01 from Sparkfun.
I have made a simple to use library for Arduino that uses the HCD or similar radio for simple character driven communication.
In addition to communicating between Arduinos it can be used to connect to the HCD or similar handsets so they may be used for controlling your own stuff.
See the library examples for basic functionality.
Check out the Arduino library.