Virtual MIDI Piano Keyboard is a MIDI events generator and receiver. It doesn’t produce any sound by itself, but can be used to drive a MIDI synthesizer (either hardware or software, internal or external).

Yoshimi is a software audio synthesizer, originally forked from ZynAddSubFX.

drumkv1 is an old-school all-digital drum-kit sampler synthesizer with stereo fx.

An easy to use, pattern based midi sequencer, a program that sends digital “notes” to software instruments such as synthesizers and samplers.

Rosegarden is a music composition and editing environment based around a MIDI sequencer that features a rich understanding of music notation and includes basic support for digital audio.