Free-Loops.com is a site that is dedicated to music loops and sound effects. They currently have around 8,000+ music loops & sound effects. Most of them are in wav, but some of them are in mp3 format.
They also offer vst plugins, a cool little forum, free multimedia software, and some music production related videos.
This site is a super cool resource for music producers, and video producers looking for some cool sound effects.
The site has been around since 2005, and seems to be updated near daily with fresh content.
Have a look and bookmark them if you are interested.