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.

Fldigi can also use libsndfile and hamlib if they are
available at build time.