Don\'t forget that DirectX is specific to Windows exclusively. That is the Microsoft\'s little \"we don\'t want to play by everyone else\'s rules, we will make up our own\" policy, which many of the windows technologies works off of. OpenGL on the other hand extends across many different platforms, from SGI\'s own systems (SGI developed OpenGL) to Windows, Linux, Mac, etc. While originally it was a misnomer (the technology wasn\'t really \"open\" in the same sense as open-source projects), I think (not 100% certian) than it has been made less proprietary & more open as of late.
There\'s also an audio version (called, funny enough, OpenAL) which is being worked on, but it hasn\'t taken hold as much, and I don\'t believe it\'s even the same group (or even a group related to the OpenGL developers) that is working on it.