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. MusE aims to be a complete multitrack virtual studio for Linux, it is published under the GNU General Public License.

v0.7 has just been released and it now supports Windows VST plug-ins!

• NEW Advanced jack audio routing possibilities.
• NEW Automation architecture for both midi and audio.
• Improved Better midi controller abstraction: available midi controller can be defined in instrument definition file (*.idf).
• Improved Integrated mixer with:
  • NEW Unlimited number of inputs.
  • NEW Unlimited number of outputs.
  • NEW Unlimited number of tracks.
  • NEW Unlimited number of aux's.
  • LADSPA effects on all track types, both before and after fader.
  • Stereo/Mono track types.
• Improved interface changes, the TrackInfo pane is used more.
• Improved Midi instrument definition files (*.idf).
• Improved Lots of customization options.
• NEW Shortcut editor.
• NEW Drag and drop import of midi files.
• NEW 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.
  • NEW Jack transport enabled.
• LADSPA effects.
• Integrated softsynth architecture M.E.S.S
  • Several internal synths available.
  • Simple architecture for adding more synths.
  • NEW 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.