I very cheap USB dvb-t receiver that can be used as a software defined radio with GNU radio running on a Raspberry Pi.
http://sdr.osmocom.org/trac/wiki/rtl-sdr rtl-sdr
DVB-T dongles based on the Realtek RTL2832U can be used as a cheap SDR, since the chip allows transferring the raw I/Q samples to the host, which is officially used for DAB/DAB+/FM demodulation. The possibility of this has been discovered by the V4L/DVB kernel developer Antti Palosaari.
Specifications
The RTL2832U outputs 8-bit I/Q-samples, and the highest theoretically possible sample-rate is 3.2 MS/s, however, the highest sample-rate without lost samples that has been tested so far is 2.8 MS/s. The frequency range is highly dependent of the used tuner, dongles that use the Elonics E4000 offer the widest possible range (64 - 1700 MHz with a gap from approx. 1100 - 1250 MHz). When used out-of-spec, a tuning range of approx. 50 MHz - 2.2 GHz is possible (with gap).
This little computer runs UNIX (Debian), just plug in keyboard, mouse (USB), Display (HDMI), SD card, Ethernet cable, and away you go! Incredibly cheap and small, and very easy to administrate with 30,000 application packages installed at the click of your mouse! Unbelievable!
http://en.wikipedia.org/wiki/Raspberry_Pi
http://www.newark.com/raspberry-pi/raspbrry-pcba/raspberry-pi-model-b-board-... RASPBERRY-PI - RASPBRRY-PCBA - RASPBERRY PI MODEL B Price: $35.00