am433 is a toolkit for developing, debugging and reverse engineering devices that use simple, packet based ISM band transmissions. Common devices that can be analyzed this way are cheap battery powered weather monitors and other wireless sensors, various remote controls and key-less access systems.
It includes a design for a simple and cheap USB connected radio receiver that can receive digital binary amplitude modulated transmissions (also known as OOK - on/off keying, 100% amplitude modulation, ASK). It uses a hybrid super-regen receiver module, which is available off-the-shelf for 433 and 868 MHz ISM bands and an audio codec from an USB dongle.
The digitalized baseband data can then be analyzed using a software demodulator that can heuristically detect and decode various encoding schemes and a patched libpcap and tcpdump.
The following blog posts contain some more background information:
I also gave a talk about this project in Kiberpipa (recording is in Slovenian language).
How to get the source
Fetch the latest version from http://www.tablix.org/~avian/am433 and consult the README file. The archive also contains hardware documentation (schematic and PCB artwork) should you want to build your own receiver.