Fldigi is a modem program for most of the digital modes used by radio amateurs today: CW, PSK, MFSK, RTTY, Hell, DominoEX, Olivia, and Throb are all supported. It can help calibrate a sound card to a time signal and do frequency measurement tests.
Definetly a good program. Woks on Linux, so that's a big plus. Good user interface. Very stable. I give it four stars and a half.