Audio software with JACK support

Ardour (software)
Ardour is a hard disk recorder and digital audio workstation application.

Baudline
The baudline time-frequency browser is a signal analysis tool designed for scientific visualization.

BRP-PACU
BRP-PACU is a dual channel FFT audio analysis tool.

ChucK
ChucK is a concurrent, strongly timed audio programming language for real-time synthesis, composition, and performance, which runs on Mac OS X, Linux, Microsoft Windows, and iPhone/iPad.

FreqTweak
FreqTweak is an open-source tool for real-time audio spectral manipulation and display.

MusE
MusE is a free software MIDI/Audio sequencer with recording and editing capabilities.

Music on Console
Music On Console (MOC) is an ncurses-based console audio player for Linux/UNIX. It was originally written by Damian Pietras, and is currently maintained by John Fitzgerald.

Music Player Daemon
Music Player Daemon (MPD) is a music player server.

Qjackctl
QjackCtl is a Qt-based tool for controlling the JACK Audio Connection Kit.

Qtractor
Qtractor is a hard disk recorder and digital audio workstation application for Linux.

Rosegarden
Rosegarden is a free software digital audio workstation program developed for Linux with ALSA and QT4.

VLC media player
VLC media player (also known as VLC) is a highly portable free and open-source media player and streaming media server written by the VideoLAN project.

XMMS
The X Multimedia System is a free software audio player very similar to Winamp, that runs on many Unix-like operating systems.