I came across a blog about sound effects, and, more specifically, free sound effects. (www.sfxsource.com) It's a pretty complex and well set up blog with a lot of features. It has many, many sound effects available for purchase, as well as several links that I can see at the moment that can help us find free sound effects. You should check it out.
The part of this blog I find most interesting, however, is the most recent post on the blog. It talks about how sometimes when we're watching a movie, sound effects that we hear don't quite match up with what we're seeing. This is because an alarming number of these sounds are actually added in post-production. I find this really fascinating, and it's a good, valid explanation to something we've probably all noticed.