With the advent of low cost but relatively powerful microprocessors, it has been possible to move several conventional radio stages from hardware into software.
No only has conventional hardware been replicated by software, but by using digital signal processing techniques, the capability of software defined radios (SDR) has expanded.
It is now possible to continuously capture a whole chunk of radio spectrum into the digital domain, process the data and select the modulation mode and bandwidth from withing the software interface. In fact, as the whole spectrum is captured, its possible to run several ‘software radios’ all listening to different channels at the same time.
With the use of a very low cost DTV-USB adapter (maybe $20) it is possible to turn your PC computer into a SDR. Obviously performance can be improved, but with a small antenna you can be listening to the Ham bands and much more. These “SDR Dongles” generally cover the frequency range of about 10MHz up to about a 1GHZ or so. Not bad for a few dollars!
If you are curious, we can help get you started. In the meantime there is quite a bit of additional information available: For example, here.