Fldigi is a "digital modem" program for Linux, FreeBSD, and Windows.  
This software allows amateur radio enthusiasts to communicate using a 
number of digital protocols, including but not limited to PSK, 
Hellschreiber, Olivia, Morse Code, and many more.  Receiving signals is 
as simple as plugging an audio cable into your line in or microphone 
port on your sound card, and transmitting can be accomplished using one 
of any number of prebuilt or homebrew interfaces which can found on the 
internet.