• Ardour Digital Audio Workstation.
• Qtractor Audio/MIDI multi-track sequencer.
• MuseScore Music notation and composition software
• LinuxSampler sampling with SFZ and GIGA files.
• TAL Noisemaker Excellent synth available as LV2 and LinuxVST plugins as well as a JACK app.
• samplv1 RNCBC's simple but very useful sampler. Available as LV2 plugin and JACK app.
• Open Octave Midi is a Linux based DAW designed to give musicians the tools needed to make composing on a computer possible, and even fun!
• Mixxx DJ tool.
• Calf audio plugin pack high quality open source audio plugins for musicians.
• Audacity sound editor
• Hydrogen drummachine
• Yoshimi Improved version of the famous Zynaddsubfx synth
• Non A set of fast, light, modular tools for music production on Linux.

These are just a few examples, more at apps.linuxaudio.org Not sure if a tool you need is available or don't know which one suits your needs best? Ask for advice and user experiences on the forums or mailinglists!

• You can find a list with audio applications for linux at the page of wikipedia.org: http://en.wikipedia.org/wiki/Linux_audio_software

• A good resource to find linux audio applications for a specific area (notation editors, drummachines etc. etc.) is the application page of linuxaudio.org, section 'Audio Applications': http://apps.linuxaudio.org/