SoundSnap is a free online sound library and community for sound designers and producers. People can upload sounds and share them with the rest of the public.
It's advertised as being mainly for Sound designers/recordists and music producers, filmmakers, web designer's and video game developers, but since photographers are increasingly getting involved in multimedia, I think we could find SoundSnap useful when needing a specific sound (or loop) for a slideshow. For instance, there are various train sounds in the library which could've been useful to photographers documenting the Central American migrants (see above post).
Is it cheating because it's not ambient sound recorded live by the photographer? Yes, perhaps some will see it this way...others won't.
(Via The Graphic Mac)