I make playable, interactive visualizers that synchronize with live music. This is one of my current ongoing side projects.
My custom software can fire off events in the game in time with the tempo of the song, as well as performing spectrum analysis on the incoming audio. It is set up with keybinds so I can control it during a performance.
My first performance with this rig was for Slime Girls at Rockage 2.0. It was a collaboration incorporating art from Pedro Silva (of Slime Girls,) Grace Voong, and videos from Max Capacity. I have used this for several other Slime Girls shows, including POW POW: A colossal night of sound and vision at GDC 2013.
I try to keep evolving this setup. When Slime Girls opened for Anamanaguchi in the summer of 2013, I debuted a new hybrid rig that uses the game visualizer as an interactive video source feeding into the VDMX video mixer. Since Unity doesn't currently support recieving input when the game window is out of focus, I created an alternative input scheme using a Wiimote connected via OSCulator and OSCuMote for Unity. Using VDMX, I can also mix in other visualizers I've created in Quartz Composer and perform more expressively using a MIDI controller.
My working title for this incarnation of the idea is "Game Visualizer 1" which is extremely boring. I hope that the experience itself is much less so!