This sort of setup can also be used to monitor aircraft squelches. I got a few Raspberry Pis during a B1G1 sale, so I have one of these setups at work and at home. I like watching the planes as they take off from the local airport and queue up for arrival, and also seeing the nightly freight flights.<p>More info here: <a href="http://flightaware.com/adsb/piaware/" rel="nofollow">http://flightaware.com/adsb/piaware/</a><p>You can feed into sites like Flightaware and FlightRadar24 and get accounts with them that normally cost a lot for free.
If people are interested in listening to signals I'd recommend checking out the ELF to VLF bands. You can, in most cases, build a receiver that operates directly on your system's sound card. These lower frequencies largely present an antenna building challenge but there's lots of interesting things down at those frequencies. Submarines, hams, and space weather.<p>The HF to EHF bands are fun but the lower frequencies, which are mostly ignored, are just as much of a gold mine for fun projects.
If you do not have one, get yourself an RTL-SDR today. This is a cheap ($10) way into radio and you will be impressed with what you can do.<p>From radio scanning to public safety radio to airplane tracking all with free software!