OsmoSDR is a 100% Free Software based small form-factor inexpensive SDR
(Software Defined Radio) project.
If you are familiar with existing SDR receivers, then OsmoSDR can be
thought of something in between a FunCube Dongle (only 96kHz bandwidth)
and a USRP (much more expensive).
For a very cheap SDR (with limited dynamic range), you can use the DVB-T
USB stick using the RTL2832U chip, as documented in rtl-sdr.
It consists of a USB-attached Hardware, associated Firmware as well as
GrOsmoSDR gnuradio integration on the PC.