The Universal Software Radio Peripheral (USRP) project is an open-source software radio system that can grab and decode anything: GPS, FM radio, HDTV, and even the signals sent by garage door openers. Like a radio Lego set, you simply point the software and antenna at a source and it begins grabbing everything within that band. You can then modify the incoming stream via software, allowing you to control just about any stream without expensive hardware.

For example, USRP's creator, Eric Blossom, and his partner Matt Ettus are working on a passive radar system that creates a map based on radio waves coming from various sources. They're selling their creations and have a daughterboard available for $550. The potential is pretty amazing, especially considering the current glut of wireless gear out there.

Product Page [Ettus]
GNU Radio Opens an Unseen World [Wired]