Music
Ardour (VST) : Record and (re-)mix soundtracks.
BCMan : BCR2000 editor
Calf Plugin pack for JACK : instruments and effects rack
Drummer-s-Gigsaw : pattern library
DrumSynth : drums synthesizer
Hydrogen : drum machine based on samples
Jack-Mixer : mix any number of Jack channels.
JSynthLib : Synthesizer editor / librarian
Muse : midi editor (should send Midi clock but I couldn't get it working)
PureData : compose effects or soundgenerators, samplers, ...
QSampler - LinuxSampler - Gigedit : play / edit GigaSampler sample libraries
Qsynth - Fluidsynth : play SoundFont sample libraries
Qtractor : midi and audio recording+plugin host+jack connections+hardware setup
Rakarrack : Guitar effects
Rosegarden : midi editor
Seq24: sequencer with Midi clock to sync my Yamaha CS2x arpeggio.
TapStart : tap to set JACK's tempo
FreqTweak : EQ and Dynamics for jack
Stretchplayer : play ogg or flac files at different speeds without changing the pitch
Tapeutape : Sampler for JACK