SC 2 is the MacOS 9 version of SuperCollider. It is the most common,
but with the inability of new computers to run MacOS 9 it will become less and less common.
There was also a free player version at the times when SC was non-free software, called SCPlay, it is not needed anymore.
If you want to use sc2 under os x, you will need to find drivers.
Midi can also be ported to Classic OMS, (didn't test it):