If you've ever been curious about software defined radio (SDR), this USB stick is the easiest way possible to have fun with a powerful, configurable receiver. Packed with the powerful RTL2832U and R820T tuner, it can tune into signals from 24MHz to 1850MHz. That means you can use a computer (with Windows, Mac, or Linux) to tune into: FM Radio, AM signals (but not AM radio), CW (morse code!), unencrypted radio signals (such as those used by many police and fire departments), POCSAG pagers, and more.
We plugged it into our windows computer and used Zadig+SDR# to poke around and listen to the wireless world around us, it's lots of fun!
Adafruit has a basic getting-started tutorial here, and there are more tutorials at these fine sites:
Comes with a very compact (but very good quality) RTL2832+R820T module with an MCX RF connector, and a basic 'DVB-T' antenna (used to tune into DVB TV signals, not available in the US but will work in Europe or any country with DVB-T broadcasts) as well as an IR remote which isn't terribly useful. The antenna seems OK for basic experimentation.
Please note! Depending on your location, listening in to some radio signals may not be legal. So consider this your warning to check before 'accidentally' tuning into naughty frequencies.