WSPR is a piece of software that enables you to participate in a world-wide network of low power propagation beacons. It enables your radio transceiver to transmit beacon signals, and to receive beacon signals from similarly-equipped stations in the same amateur band. Because participating stations usually upload spots that they receive in real time to a web server, you can find out within seconds of the end of each transmission exactly where and how strongly it was received, and even view the propagation paths on a map.
(From the http://www.g4ilo.com/wspr.html web site)
W4HOD will have a guest speaker in February to talk about this very subject. Richard Robinson (AI4RY), will present the program on WSPR.