Ardour is a digital audio workstation that runs natively on OS X. You can use it to record, edit and mix multi-track audio. Produce your own CD's. Mix video soundtracks. Experiment with new ideas about music and sound. Generate sound installations for 12 speaker gallery shows...
Ardour capabilities include: multichannel recording, non-destructive editing with unlimited undo/redo, full automation support, a powerful mixer, unlimited tracks/busses/plugins, timecode synchronization, and hardware control from surfaces like the Mackie Control Universal. If you've been looking for a tool similar to ProTools, Nuendo, Pyramix, or Sequoia, you might have found it.
Above all, Ardour strives to meet the needs of professional users. This means implementing all the "hard stuff" that other DAWs (even some leading commercial apps) handle incorrectly or not at all. Ardour has a completely flexible "anything to anywhere" routing system, and will allow as many physical I/O ports as your system allows. Ardour supports a wide range of audio-for-video features such as video-synced playback and pullup/pulldown sample rates. You will also find powerful features such as "persistent undo", multi-language support, and destructive track punching modes that aren't available on other platforms.
Full list of changes available here