FreeSFX offers more than 3000 free sound effects and music tracks. All downloads are mp3:s and to get access you have to become a member.
Freesound.org has been around since 2006 and over the years they have collected over 100,000 sound effects and music tracks. The uploader gets to choose between different licenses so make sure you select files that properly suits your project.
Hiss and a Roar
Tim Prebble does not only sell sound effect libraries, he also offers parts of them for free. The only thing he asks in return is that you subscribe to his newsletter.
The free sound effects at PacDV seems to be very similar to those at SoundJay. You can download them directly without registering. The sound effects are either mp3 or wav.
Partners in Rhyme
The Partners In Rhyme site has been around since 1996. Their main focus is to sell royalty free music and sound effects but they also have a free section. The site is somewhat chaotic and quite tricky to navigate.
The audio at Sound Jay seems to be similar to those at PacDV. Every free sound effect is available in both wav and mp3. No registration necessary.