I've been using Audacity since about 14 years ago, and it's truly come a long, long way. Now if we can just sort out the PulseAudio/ALSA mess, we'll be able to make some sweet music indeed.
This is not exactly the right thread perhaps... but MuseScore is an excellent musical notation software available for Linux systems. I switched a friend from Sibelius to MuseScore about three or four years ago and for his needs he says he likes MuseScore better (than the version of Sibelius going at that time). Admittedly he is not a power user, but he does draft up a surprising amount of scores in very little time...