Thought it might be intereating to know what your favourite open source audio programs are...don't forget to post a link!

1st on my list is Audacity. great for recording Audio (from any source)to you Hard disk and then editing.
http://audacity.sourceforge.net/

I also use mixxx on the odd occasion I have to DJ
http://www.mixxx.org/

MuseScore. Doesn't get much use, but cost much less than Sibelius!
http://musescore.org/en