My programming language and graphical environment for MIDI, which used to be called Keynote. AT&T is making it available in complete source code form for non-commercial use.
The download site for KeyKit provides a summary of the language and user interface features, and allows you to download documentation, source code, and executables for Windows, Linux, and the Mac.
When the Standard MIDI File format was first standardized, I wrote this midifile library that reads the format. Michael Czeiszperger (firstname.lastname@example.org) added support for writing.
Glib is my ancient but possibly still useful generic MIDI librarian/editor with source code, Windows executable, and support for the TX81Z, DW8000, and Waldorf Microwave.
STEVIE is my little vi clone that grew (after more than a decade of Open Source development) into the widely-available and widely-used VIM editor.
NthEvent is a little XML server for MIDI and iGesture pad events, useful with Flash-based software (like Onyx - see http://www.onyx-vj.com ) that doesn't have direct access to those input devices. Note: support for NthEvent was present in Onyx 2.*, but was broken and subsequently removed in Onyx 3.*. Unpack this zip file and read the readme.txt file for instructions on running it.
nosuch.vstutil is a Python module with classes that lets you play with VST audio things on Windows. It uses portaudio for audio device interfacing and libsndfile.dll for reading sound files. It does not currently allow interaction with VST GUI interfaces, but does allow interconnection of VST plugins and manipulation of their parameters. It is not well-packaged (for a Python module) - you have to unzip it, read the readme.txt file, and install it manually. Several demo examples are included. It was originally developed for my Radio Free Quasar installation. Here are some powerpoint slides describing it.