One of the most oft-tapped areas of content are files that are in the public domain. I'm not a lawyer, so I won't attempt to define the term "public domain", but here's a link to the Wikipedia entry that does a better job than I would.
Files in the public domain are a great source of content for blogs and podcasts. Copyright has lapsed on these files so that there are no restrictions on how the content can be used.
However, in the US at least, it is sometimes difficult to figure out whether a file is in the public domain or not.
Eric Giguere has a pretty good article on this topic. Eric primarily deals with Adsense and stuff, but this entry has some good info and some good links to sort out how to determine if something is the public domain.