When creating music, visuals many times come into play.... Visual programming vvvv: a multipurpose toolkit : vvvv : a multipurpose toolkit - a toolkit for real time video synthesis. Processing 1.0 (BETA) - open source programming language and environment for people who want to program images, animation, and interactions. Context Free Art - generates images from written instructions called a grammar. NodeBox | Home - a free Mac OS X application that lets you create 2D visuals (static, animated or interactive) using Python programming code and export them as a PDF or a QuickTime movie.
Interactive art Dave: The Interactive Music Video Installation - cool art project which lets people interact with light pillars to change things within the video shown on multiple screens.
Interfacing GAINER.cc | About / About GAINER - Gainer lets the user handle sensors and/or actuators with a PC on various programming environments such as Flash, Max/MSP, Processing and so on. Open source + Open hardware.
Of course midi and network control provide great interfaces, I have another page for that... MIDI and OpenSound Control- network OSC and all things MIDI reactable - vision pattern recognition based using reactable software TUI protocol. (open software!) openFrameworks - looks like a neat C++ library for creative coding. (not released yet....)