MusE is a multitrack virtual studio for Linux that has support for sequencing of both MIDI and audio and has, among other things, support for LADSPA, Jack and ALSA.

• Advanced jack audio routing possibilities.
• Automation architecture for both MIDI and audio.
• Better MIDI controller abstraction: available MIDI controller can be defined in instrument definition file (*.idf).
• Integrated mixer with:
  • Unlimited number of inputs.
  • Unlimited number of outputs.
  • Unlimited number of tracks.
  • Unlimited number of aux's.
  • LADSPA effects on all track types, both before and after fader.
  • Stereo/Mono track types.
• interface changes, the TrackInfo pane is used more.
• MIDI instrument definition files (*.idf).
• Lots of customization options.
• Shortcut editor.
• Drag and drop import of MIDI files.
• Drag and drop import of wave files.
• Standard MIDIfile (smf) import-/export.
• RTC (Real time clock) use for rock solid timing.
• Advanced arranger with track and part abstractions.
• MIDI editors:
  • Pianoroll.
  • Drum editor.
  • List editors.
  • Controller editors.
• Realtime editing of "everything".
• Unlimited number of open editors.
• Unlimited undo/redo.
• Realtime and step-recording.
• Multiple MIDI devices.
• Unlimited number of tracks, both MIDI and audio.
• Audio playback/recording.
• Sync to external and internal devices/applications:
  • MTC/MMC.
  • MIDI Clock.
  • Master/Slave.
  • Jack transport enabled.
• LADSPA effects.
• Integrated softsynth architecture M.E.S.S
  • Several internal synths available.
  • Simple architecture for adding more synths.
  • support for industry standard Windows VSTi softsynths.
• JACK enabled.
• ALSA sequencer client.
• LASH enabled (formerly LADCCA).
• Raw MIDI device usage through ALSA.
• XML based project files.
• XML based configuration files.
• project file contains complete app state (session data).
• Application spanning Cut/Paste Drag/Drop.
• uses C++, QT3.2 GUI Library, STL.

And the most important feature for a free application:

• GPL Licensed.