Audulus

This is such a good modular environment for learning synthesis, making music and effects and, now, in Audulus 4, creating multi-touch control surfaces (with LUA scripting) for internal and/or external instruments over MIDI. No other modular environment is so immediate and deep at the same time, in my opinion. It makes for a great teaching tool as well, for both scriptable and graphical approaches to synthesis. The community is also very active and helpful.

It has its limitations. Audio and MIDI settings are suprisingly modest, and some parts of the user interface could be optimized for better usability imo. The desktop app does not host VST/AU plugins, and being an AUv3 has its limitations. But these limitations does are trivial when you let Audulus be what it wants to be; a playground for learning and building instruments and effects, completely from the ground up, or in combination with modules built by the active and growing community.