-> Feel free to use this thread to post links to useful websites! <-
The Digital Artists Handbook features an article written by Thor Magnusson with an impressive list of open source (FLOSS) audio tools:
http://www.digitalartistshandbook.org/sound
He categorizes the tools as follows:
Audio Editing
Sequencing
Score writing
Virtual Instruments
Sound Feature Analysers
Patchers - Algorithmic/Interactive/Generative Composition
Note Flight
By Born City RatAn online program - you can compose and then export audio. Some may find this useful.
Completely free.
http://www.noteflight.com/