I just finished up my wireless garage door monitor. I live upstairs above my garage (so I can't see it) and thought it would be useful to be alerted if I left my garage door open. I used an RF Link transmitter/receiver pair from SparkFun, an attiny2313 on the transmitter side, and an atmega16 on the receiver side. The atmega16 is most definitely overkill, but it's what I had laying around.
The part of the system that resides in the garage consists of a transmitter board with the RF Link and attiny2313 connected to a small breadboard with a magnetic switch mounted on it. When the garage door is open, two neodymium magnets stuck to the garage door cause the magnetic switch to close. The attiny2313 then relays this information via the RF link to the receiver upstairs. The breadboard was mounted to the garage door opener with another set of neodymium magnets for convenience.
The receiver board has a green LED that toggles when a 'good' packet is received and a red LED that toggles when a mangled one is received (this is very rare). When the garage door is open, the piezo buzzer beeps a few times and then goes into a continuous buzz. The button on the board can be pressed to disable the buzzing for 10 minutes. Once this gets annoying, I will probably change it such that the buzzing only starts if the door has been left open for a certain period of time.