Ardour

From the Ardour home page: "Ardour is a digital audio workstation. 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. Have Fun."

Ardour is indeed a Digital Audio Workstation (DAW) and much more. It is comparable to the best proprietary DAWs commonly used in professional recording, and has alleged advantages over them, many due to its being built from open source software. As reported on Ardour's web site, Ardour's capabilities include:
 * multichannel recording
 * non-linear, non-destructive region based editing with unlimited undo/redo
 * full automation support
 * a mixer whose capabilities rival high end hardware consoles
 * lots of plugins to warp, shift and shape your music
 * ability to be controlled from hardware control surfaces at the same time as it syncs to timecode

Getting started:
Ardour does not directly use audio interface cards; rather, it operates through JACK. Therefore, any soundcard that can be used with JACK can be used with Ardour. For professional quality work, the Ardour community tends to prefer the RME Hamerfall Series or the M Audio Delta series of cards. There are also less expensive cards that should work well, expecially those based on the ICE1712 or ICE1724 chipsets, including those from M Audio and Terratec. In general, since JACK works with ALSA, cards that have ALSA drivers (see the link to the soundcard matrix below) should work.
 * Choosing a sound card
 * low latency
 * hardware monitoring
 * Getting a good kernel
 * Use kernel 2.4 + lowlatency patches or kernel 2.6 compiled with Preemptible kernel option.

The JACK utility is run from the command line interface. Easier control is available through the very complete GUI interface provided by QJackCtl. The main interface window contains controls for starting and stopping the JACK server, transport controls, and buttons for opening windows for messages, status, client connections, virtual patchbay, and general JACK setup.
 * JACK
 * determine the best options to run jackd.
 * Using QJackCtl to launch Jackd via GUI
 * dealing with xruns
 * LADSPA plugins