http://www.taylorhayward.org/recordings.htm

http://www.publicdomain4u.com/

http://www.royaltyfreemusic.com/free-music-resources.html

http://www.musopen.org/music_overview.php

And last but not least istockphoto – the well-known stock photography site – has a whole series of audio files. Not free, the pieces cost $5-25 each. But there is a vast range of audio on the site. There are collections of files for themes in these ‘lightboxes’ or search by keyword for the sound you need:

http://www.istockphoto.com/audio

Just heard (10/28/11) that Vimeo has a Music Store licensing thousands of tracks for videos and podcasts.

Another low-fee resource:

http://www.productiontrax.com/music.php

And another recommended site:

http://www.arthurstead.com/About_Us.htm

Ian, thanks for making this so simple.