WSPR has been around since 2008, and is a computer program used for weak-signal radio communication between hams. The program was initially written by Joe, K1JT. The program is designed for sending and receiving low-power transmissions to test propagation paths on the MF and HF bands.
WSPR implements a protocol designed for probing potential propagation paths with low-power transmissions. Transmissions carry a station's callsign, four figure Maidenhead Grid locator, and transmitter power in dBm. The program can decode signals with S/N as low as -28 dB in a 2500 Hz bandwidth.
Stations with internet access can automatically upload their reception reports to a central database called WSPRnet, which includes a mapping facility, which is great as you can visually see where your signals are being heard, or what signals you are hearing.
While watching WSPR is about as exciting as watching paint dry, and it's definitely not for everyone, it is still very interesting to leave it running for a few hours and come back and see where your 5 watts have been heard.
The evening of Monday, December 7th, saw the following stations heard from this QTH, using my VE3FCT callsign:
And here's the European stations that I was hearing:
Not too bad for 5 watts and a 148' long wire antenna.