Also tangentially relevant: In amateur radio, soundcard "A"FSK (A for audio) is super popular. The most popular are the time-synchronous WSJT-X modes (JT65, JT9, FT8, etc). It's super fascinating from a technical robustness point of view, especially the FT8 mode. The program is authored by a Nobel laureate in physics, and a very active radio ham, K1JT.
There are dozens of other time-asynchronous modulations supported by fldigi, like the 2nd popular PSK31 and RTTY. And morse code too :) http://www.w1hkj.com/
Just a small nit, most of those are FSK but PSK31 and it's variants are actually Phase Shift Keying[1] which is pretty cool in its own right. Rather than changing frequency it's a shift of phases when combined with a 90' signal(often called quadrature, hence I/Q signal you see in SDRs) you can encode more than one bit per symbol.
Also forgot to mention it but an even wider use of AFSK than FT-8 and it's WSJT cousins is AX.25/APRS over VHF. Sadly it's not nearly as advanced as FT-8 since it most commonly uses 202 Bell tones and a blazing 1200bps. There's been other digital formats but the whole situation is pretty fragmented.
The infrastructure side of APRS is pretty nifty, you have sites like aprs.fi[1] that let you see the current state of the world in near realtime anywhere an I-Gate is present.
Every time I see websdr.org mentioned, I supply this caution:
I just want to caution folks about visiting this site. You could find yourself getting sucked into a years-long obsession with RF and software defined radios. Pretty soon you'll have dozens of RTL SDRs strewn about your desk, coax tumbleweed snaring your feet and six iterations of a quarter-wave ground plane that still don't work quite as well as the first one you ever built.
https://physics.princeton.edu/pulsar/k1jt/wsjtx.html
Tune to any of the FT8 frequencies on http://www.qsl.net/sv1grb/psk31.htm using almost any receiver on http://websdr.org/ - 20m in the daytime, and 40m during evenings is usually full of activity.
There are dozens of other time-asynchronous modulations supported by fldigi, like the 2nd popular PSK31 and RTTY. And morse code too :) http://www.w1hkj.com/