Special thanks to Aaron M. for the very entertaining and well-written email. You gotta respect a mature argument like his. You can see a list of all the songs that Led Zeppelin copied from other artists here. And you can listen to one of the many songs that they did not steal on the MUSIC page, but I warn you, it's probably shit.
In music there are several sayings like "there's no such thing as an original idea," or "every song has been done before." These imply that every idea is simply a rehashing, or sometimes complete copy, of an older one. As someone who writes music himself, I can understand why this happens so often. Many times have I written what I thought was an awesome and original guitar riff, only to realize that I was just playing the same riff as a song I heard earlier that day. The temptation to use that riff and simply change the lyrics, hoping no one will notice, is sometimes too great. Usually when that happens I end up just learning the original song and playing it as a cover, but then I'm like, "Damn, I could've sworn I was onto something there!"