Is there a list somewhere? Might be nice to compile a list here.
I'm assuming mostly games but also music and educational software made use of it.
The Apple Vault has a fairly comprehensive list of software titles which supported the Mockingboard.
I do see several music programs in the list:
Bank Street Music Writer
Music Construction Set
And of course the list of games is quite long:
Adventure Construction Set
Apple Cider Spider
At the Gates of Moscow 1941
Battle Cruiser SMS-2000
Crowther and Woods Adventure
Crypt of Medea
One on One
Spy Strikes Back
Tactical Armor Command
Although based on this Atari Age discussion thread, some of these titles work better than others, and some of them (G.I. Joe, Lancaster) not at all.
In 2005 ReactiveMicro.com re-introduced the Mockingboard. Their manual can be found here, and has a similar list of supported titles.