Correspondance is an app I made back in 2002 using Director and Flash. It generates animated visuals from realtime MIDI note input (specifically pitch, velocity, and duration). Correspondance has a number of interpretation paradigms, mapping the MIDI input to various visual characteristics (x/y position, size, color, etc).
Correspondance also has a networked option where two performers can link to each other. Each performer sees the animation generated by the other player's musical performance. An audiovisual feedback loop can be created if each performer treats the visuals an an animated graphic score.
Unfortunately the app only works on Mac OS 9 so it's not currently available.