xphat - http://www.equalarea.com/paul/xphat/ A MIDI slider/fader box for Linux/X.
MIDI programming in Linux - http://www-ccrma.stanford.edu/~craig/articles/linuxmidi/ Text with C++ examples by Craig Stuart Sapp.
Random Phase Music Generator - http://phase.sourceforge.net/ Generates Steve Reich-like music randomly and puts into MIDI Files
Brahms - http://brahms.sourceforge.net/ KDE Music System - A Cubase-like music system for linux
MIDI-Perl - http://www.speech.cs.cmu.edu/~sburke/pub/perl_midi/ 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.
The Computer Music Project - http://www.cs.cmu.edu/afs/cs.cmu.edu/project/music/web/music.software.html 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.