Freesound is one I've used before, with pretty excellent results:
http://freesound.iua.upf.edu/
I agree with the "pepper" comment below - personally, I can't stand when someone gets a little too SFX happy, and every little flash, frame push, etc... has some massive sound effect tied to it! All of a sudden, your nice little package about foie gras now sounds like "Transformers"... It really pulls you out of the story.
When I put in sound effects to go with transitions, I like to keep them relatively low in the mix... if the sound effect is noticeable, it's probably too loud! There is a region between "inaudible" and "completely noticeable", which is where a lot of these SFX belong....
Christian Glawe
editor/compositor
www.christianglawe.com
Read my blog: http://blogs.creativecow.net/blog/111
Pain is temporary... film is forever.
