MultiMode
MultiMode allows you to decode and transmit morse code (CW), RTTY, FAX, SSTV, SITOR-B, NAVTEX, PACKET, ACARS, PSK31, ALE, and many other modes on your Macintosh, without any extra hardware.
ftp://ftp.blackcatsystems.com/pub/MultiModeOSX.zip - 899Kb - Download - Screenshot

SeaTTY
A program to receive weather reports, navigational warnings and weather charts transmitted on longwave and shortwave bands in RTTY, NAVTEX and HF-FAX (WEFAX) modes. Software also can automatically save NOAA Weather Radio SAME voice messages (NWR SAME) and them digital headers. ... None additional hardware is required - your need only receiver and computer with a sound card.
http://www.dxsoft.com/seatty.zip - 2Mb - Download - Screenshot

TrueTTY
A program for amateur radio digital communications via a sound card. Supported modes are RTTY (Baudot code), ASCII (7 or 8 bits), PSK31 (BPSK and QPSK), AMTOR-FEC (SITOR-B, NAVTEX), MultiFSK-16. HF-PACKET and UHF-PACKET (AX25) are supported in KISS-TNC emulation mode. ... It make fast and convenient frequence control, has a lot macros for tramsmitting. ... Can cooperate with RZ4AG AAlog logger. ... Optionally you can use simple circuit fo PTT-control. ... SELFEC SITOR and DTMF-code decoding is also possible.
http://www.dxsoft.com/truetty.zip - 1Mb - Download - Screenshot