I use my RF 434MHz Sniffer to detect packets of data sent from RF 434MHz devices such as LightWaveRF remote controls or my British Gas RC2 wireless thermostat.
I use a 3.5mm audio jack to feed the data into the Line-In socket on a PC sound card, and analyse the data using Audacity.
The gadget is very simple. It is a small box with the audio jack socket and a switch. The box contains a 3 AA battery pack and a mini breadboard with an RF 434 MHz receiver from Cool Components and a simple voltage divider circuit to produce a waveform with a maximum 1v magnitude, as required by the sound card line-in.
I based this on articles such as Capturing IR and RF Codes.