This wireless temperature sensor project uses an XBee, breakout board, and simple power supply to transmit temperature data to an Arduino base station. This looks like it could be easily expanded into a whole house monitoring system. [via Arduino.cc]
I decided to explore the more advanced features of XBee radios by building a remote temperature sensor. You can get quite a bit of control over an XBee radio without a microcontroller at all. You can configure the radio to send sensor readings at particular intervals when it detects changes on certain input pins.