The General Instrument AY-3-8910 was a multipurpose sound chip that was later licensed to Yamaha, who produced it as the YM2149F.  Though there were several differences between the GI and Yamaha chip, the functionality remained the same.

These two chips would spawn off a large family of other chips, with only minor modifications.

The AY family had 3 independent, programmable sound channels.  This allows any channel to play a variety of different sounds.  There are some limitations to this however.  For example, there is only one noise generator available for use by any channel.  The chip also had a built in waveform based envelope generator, though this was very awkward to use as a volume envelope.  Because of this, it would frequently be used as an audible sound oscillator to create the trademark "buzzy" sound of the AY.  And like noise, only one instance of this sound can be played at a time.  

In hardware, the AY could even be coaxed to play PCM sound with additional CPU help.

'[#[Trackers]
'[[maxYMiser] (Atari ST) (*.snd) '[l[http://www.preromanbritain.com/maxymiser/[link] 
'[[Vortex Tracker II] (Windows) (*.snd, *.sndh) '[l[http://bulba.untergrund.net/vortex_e.htm[link]

'[#[Players]
There are plugins for both xmplay and winamp but they don't seem compliant with Vortex Tracker II exports.  Fortunately the Vortex peeps have their own player :
'[l[http://bulba.untergrund.net/emulator_e.htm[emulator_e] (Windows)