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.
AlsaModularSynth is a realtime modular synthesizer and effect processor. It features MIDI controlled modular software synthesis, realtime effect processing with input capture, full control of all synthesis and effect parameters via MIDI, an integrated LADSPA browser with search capability, and JACK support.
Khronosphere is an audio recording tool. Its special feature is a buffer that keeps capturing audio even before the actual recording is started. This enables the user to prospectively catch any interesting sound that normally would have been lost. The buffer size is freely definable by the user. The software itself places no restrictions; the only limits are the resources of the host system. Khronosphere is very similar to and inspired by Jack Time Machine. They both have the same basic concept, but Khronosphere supports larger buffer sizes.