Virtual MIDI Piano Keyboard: a MIDI events generator and receiver

TuxGuitar is a free and open-source tablature editor.

An Audio/MIDI multi-track sequencer

A fluidsynth Qt GUI Interface

A modular DAW for Linux

MIDI Sequencer and Notation Editor with Audio Support

SysEx Controls for Linux

Your hardcore loop machine

Drumstick MIDI Monitor is a MIDI monitor for Linux using ALSA sequencer

Drumstick MIDI Metronome based on the ALSA Sequencer

Sample and MIDI device manager