WSPR stands for WEAK SIGNAL PROPAGATION REPORTER. This is quite an amazing program, along with its associated website ( http://wsprnet.org/ ) work together as one to build a huge propagation database from all over the world, as the programs spots get uploaded to the website and displayed upon a map of which ever band your using.
Have a look at the link to "WSPRing Around The World" article from QST magazine Nov 2010. You can also download the WSPR program from this page here http://physics.princeton.edu/pulsar/K1JT/wspr.html
WSPR mode is extremely practical and purposeful package. Especially for the busy person who doesn't have time to sit and call CQ all day. Not only does WSPR seek out extremely weak propagation paths it does random calling for you at an interval that you select. Results can be viewed at a later date. Its brilliant.
My favourite WSPR bands are 160m, 10m, 6m, 2m.
LOG of WSPR stations spotted or who spotted me on - 160m - 6m - 2m