An RTL SDR is great for many applications including general radio scanning, air traffic control, public safety radio, ADSB, AIS, ACARS, VDL2, trunked radio, P25/DMR digital voice, TETRA, POCSAG, weather balloons, APRS, NOAA APT or METEOR M2 weather satellites, Outernet, Inmarsat, Iridium, GSM, Milsatcom, radio astronomy, meteor scatter monitoring, DAB, or for use as a low cost panadapter with a traditional ham radio.
Just search our RTL-SDR Blog website (sorry we cannot link to it here due to eBay rules) for your application of interest to see some ideas.