Brahms: KDE Music System - A Cubase-like music system for linux
GN MIDI Solutions: Some MIDI utilities including source code
MIDI & Sound Applications:
MIDI programming in Linux: Text with C++ examples by Craig Stuart Sapp.
MIDI-Perl: A suite of Perl OOP modules which allows MIDI files to be read in and manipulated, or composed anew, and written out as MIDI files.
Random Phase Music Generator: Generates Steve Reich-like music randomly and puts into MIDI Files
The Computer Music Project: Carnegie-Mellon presents Nyquist, a sound composition language and CMU MIDI Toolkit - a collection of software for writing interactive MIDI software in C, for Linux, DOS and Amiga.
xphat: A MIDI slider/fader box for Linux/X.