Music:
- http://www.newgrounds.com/audio/
When into Techno, DNB and stuff like that I found the Newgrounds Audio Portal to be a good source (ParagonX9 etc.). It of course features other genres too... - http://www.ocremix.org/
Remixed game music. Third party usability probably is questionable... - http://www.musopen.com/
PD music, mostly instrumental. - http://www.jamendo.com/
- lots of CC-by-SA, -NC and what not. You have to check the particular licensing though, since almost every possible CC license is covered. - http://altermusique.org/
French site featuring free albums & songs. - http://freemusicarchive.org/
Lots of free music from various genres and artists. - http://www.incompetech.com/m/c/royalty-free/
Music by Kevin MacLeod covering various genres. - http://www.openmusicarchive.org/
Music from different gernes and artists. - http://opsound.org/index.php
Yet another site for music under the Creative Commons license. - http://www2.fanaticalproductions.net:8080/
(Metal) music by Marc Pullen.
An excellent resource site would be Freesound.org which features tons of sounds. Other sources would be: