Respectrable
Although most Ableton Live users probably only use it to make music, it was also one of the first Digital Audio Workstations (DAW) to offer a fully programmable API and probably still offers the best live performance abilities of any DAW.
Live officially supports programmatic access to every one of its features through the Live Object Model (LOM).
As is shown in the first figure, the LOM can be a bit of a maze because of the large number of parameters available in Live.
There is a learning curve to figuring out how the object hierarchy works but, once figured out, each and every property of a Live set is programmable.
The repeated necessity of a scriptable, real-time audio engine, and specifically one that could be used wirelessly through Open Sound Control (OSC) resulted in the creation of Respectrable.
Respectrable is a Max for Live device that facilitates easily accessing the Live Object Model through Javascript or Max objects.
The video in this post shows a very simple example of bidirectional control between Live and a graphical application.