When I was in college, I took a film class. When it came time for postproduction on our action movie, my group and I scrambled to come up with some decent sound effects and ambient music without having to go back and rerecord new sounds, or create something catchy in Garageband. Most of the Web resources we ran into just didn't cut the mustard when it came to finding copyright credit. Our solution was to use the small assortment of audio CDs from the school library that contained short, public domain sound effects--a process that required first loading and unloading CDs into our computers, then ripping the tracks into various formats. This may not seem like a lot of work, but getting access to those CDs in the first place was the hardest part. My guess is that the novice podcaster, and other aspiring filmmakers have run into similar problems when starting out.
Enter SoundSnap, a database of public domain sound effects and audio clips that have been tagged, categorized, and created by users. Previews play right when you click the button, and you can download them right away in multiple formats (MP3, AIFF, and WAV).
There are all sorts of ways to track down sounds, either by category or tag. The categories are intelligently broken down into 16 thematic chunks, then separated again so you can drill down to find what you need. The tag cloud also makes it pretty easy to see what's the most prominent set of sounds per genre.
The social aspect is also fascinating. Registered users can show off their work on their profile, and keep a list of their favorites from other users. They also get their own tag cloud, and optional contact information--which could be useful if you're trying to credit them, or get in touch.
I really wish I had a service like this around when we were wrapping up our film. It really does just make things a whole lot easier. If you're into these services, it's also worth checking out The FreeSound Project, The Internet Audio Archive, and ccMixter.