TuxGuitar is a free and open-source tablature editor.

Virtual MIDI Piano Keyboard: a MIDI events generator and receiver

An Audio/MIDI multi-track sequencer

A fluidsynth Qt GUI Interface

A modular DAW for Linux

MIDI and Audio Sequencer and Notation Editor

Your hardcore loop machine

SysEx Controls for Linux

Drumstick MIDI Metronome based on the ALSA Sequencer

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

Sample and MIDI device manager