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:










