ELSIE - The Windows program for electrical filter design and analysis
WSPR-net - The Weak Signal Propagation Reporter Network is a group of amateur radio operators using K1JT's MEPT_JT digital mode to probe radio frequency propagation conditions using very low power (QRP/QRPp) transmissions. The software is open source, and the data collected are available to the public through this site.
WSJT-X - web site for WSJT-X with good instructions