jack-tuner is a musical instrument tuner that uses the JACK audio connection kit. It is a command line program, and it accepts many commands. The important ones are the tone and volume commands, which can produce any audible tone from 20hz to 20khz at a volume from 0 (silence) to 1 (loudest). It can also produce notes (for example, "tone D#" produces the note D#) over several octaves (for example, "tone C octave -1" produces the note C an octave below middle-C). It can also transpose notes (for example, "tone C > 5" produces G). There are commands to do microtonal tuning too. With multiple jack-tuner instances, you can tune chords. jack-tuner does not measure the note that an instrument is playing; instead you play the desired note with jack-tuner and tune your instrument by ear.
din is a software musical instrument and audio synthesizer. Bezier curves are used to draw and sculpt waveforms, create gating and modulation (FM and AM) patterns, and create delay feedback and volume patterns. You can also create an unlimited number of drones and sculpt their waveforms. It uses JACK to output audio, and supports MIDI, OSC and IRC bot for input. din can be extended and customized with Tcl scripts.
LinuxBand is a GUI front-end for MMA (Musical MIDI Accompaniment). Type in the chords, choose the groove, and LinuxBand will play a musical accompaniment for you. It’s an open source alternative to Band-in-a-Box featuring: an easy to use graphical interface, an open and well-documented data format, and output to JACK MIDI to facilitate co-operation with other audio applications.