QjackCtl is a simple Qt application to control the JACK sound server daemon, specifically for the Linux Audio Desktop infrastructure. It is written in C++ around the Qt4 toolkit for X11, most exclusively using Qt Designer. It provides a simple GUI dialog for setting several JACK daemon parameters, which are properly saved between sessions, and a way to control of the status of the audio server daemon. With time, this primordial interface has become richer by including an enhanced patchbay and connection control features.
Qsynth is a GUI front-end application for Fluidsynth. Fluidsynth is a command-line software synthesizer based on the Soundfont specification. Qsynth was built with the Qt4 toolkit using Qt Designer. Eventually it may evolve into a softsynth management application that allows the user to control and manage a variety of command line softsynths, but it currently just wraps the excellent Fluidsynth program.
Qtractor is an Audio/MIDI multi-track sequencer application. The initial target platform is Linux, using the infrastructure of the Jack Audio Connection Kit (JACK) for audio and the Advanced Linux Sound Architecture (ALSA) for MIDI. It is meant to be a fairly-featured Linux desktop audio workstation GUI, specially dedicated to the personal home studio.