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 Clarinet Master Flute Master GuitarMaster Music Construction Set Music Star Saxophone Master Trumpet Master
And of course the list of games is quite long:
Adventure Construction Set Airsim-3 Apple Cider Spider At the Gates of Moscow 1941 Auto Gyro Battle Cruiser SMS-2000 BerZap! Bouncing Kamungas Broadside Crime Wave Crowther and Woods Adventure Crypt of Medea GI-Joe Lady Tut Lancaster Legionnaire Lethal Labyrinth Maze Craze Microbe Night Flight One on One Operation: Obliteration Pitfall II Popeye Rescue Raiders Silent Service Singing Master Skyfox Spy Strikes Back Tactical Armor Command Thunderbombs Ultima III Ultima IV Ultima V Under Fire Willy Byte Window Zaxxon ZooKeeper
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.