Plenty of my projects include sounds from FreeSound.org. This site is dedicated to linking recording enthusiasts with those who require sounds of all sorts in their projects. Filmmakers, DJ's, Music Producers, people creating presentations... etc.
If you need a sound, this is the first place I suggest checking. Most sounds carry the attribution license, which means you can use the sound for free as long as you offer the author credit.
When I say "sound" in this context it can mean anything from a snare drum hit to the sound of wind howling to the sound of a Tibetan Singing Bowl . These sounds may be actual field recordings or synthesized. There are packs of sounds as well, and one excellent sound pack is this acoustic drum pack .
If you produce any sort of music I strongly suggest becoming a member of FreeSound.org .