Rtl-sdrs are great, probably one of the best accidental discoveries I've seen. So many things being sent out from 0-1.5GHz that you can sniff. One that I see often are the unencrypted pagers broadcasting patient details, hotel security information etc. Then you've also got amateur radio, air traffic control, GSM, key fobs, weather stations, satellites, all within the reach of anyone with $15 to spare.