VST is an interaction protocol for music generation software. Here's a neat, very simple VST host called VSTHost. The current version 1.5.4 is closed source, with the latest open source version at 1.16k. (He got tired of people ripping off his features without crediting him - can't blame him for that.)
I ran across this at all because the Windows-native version of ZynAddSubFX is VST-capable and bundles VSTHost. ZynAddSubFX is a Unix music mixer, open-source. I ran across it from a short article summarizing Linux music software, which I turn had found on a search for "open source composer software" after seeing references to Mario Paint Composer for building 8-bit game music, along with a masterwork of the medley art here. (That must have taken forever to do.)
So that was my recent music software trajectory.