Cadence (JACK Toolbox)
Cadence is a set of tools useful for audio production.
This is the main app. It performs system checks, manages JACK, calls other tools and make system tweaks.
Homepage and Documentation: http://kxstudio.sf.net/cadence

Catia (Patchbay)
Catia is a JACK Patchbay, with some neat features like A2J bridge support and JACK Transport.
It's supposed to be as simple as possible, so it can work nicely on non-Linux platforms.
Homepage and Documentation: http://kxstudio.sf.net/catia

Claudia (Session Manager)
Claudia is a LADISH frontend; it's just like Catia, but focused at session management through LADISH.
It has a bit more features than the official LADISH GUI, with a nice preview of the main canvas in the bottom-left.
It also implements the 'Claudia-Launcher' add-application style for LADISH.
Homepage and Documentation: http://kxstudio.sf.net/claudia

Renoise (Tracker, non-free)
Renoise is a complete, expandable Digital Audio Workstation (DAW) with a refreshing twist.
It lets you record, compose, edit, process and render production-quality audio using a tracker-based approach.
It features a wide range of built-in audio processors, alongside support for all commonly used virtual instrument and effect plug-in formats.
Homepage: http://www.renoise.com
User Manual: http://tutorials.renoise.com/wiki

SunVox (Tracker, non-free)
SunVox is a small, fast and powerful modular synthesizer with pattern based sequencer (tracker).
It is a tool for those people who like to compose music wherever they are, whenever they wish.
On any device. SunVox is available for Windows, Linux, Mac OS X, Windows Mobile, PalmOS, Maemo, Meego, iOS and Android.
Homepage: http://www.warmplace.ru/soft/sunvox/
User Manual: http://www.warmplace.ru/wiki/sunvox:manual_en