GNU Radio is a free, open-source software development toolkit that provides signal processing blocks to implement software defined radios. It can be used with external RF hardware to create software-defined radios, or without hardware in a simulation-like environment.

RTL-SDR is a very cheap software defined radio that uses a TV tuner dongle based on the RTL2832U chipset. Essentially, this means that a cheap $20 TV tuner can be used as a computer based radio scanner. A scanner capability that would have cost hundreds of dollars just a few years ago.