I would love to know the "famous" versions of some the songs by the songwriters you guys post.
John Prine, for instance, was covered by Bonnie Raitt doing Angel From Montgomery (it's worth collecting all the beautiful versions of that song out there, but I think Bonny is the only truly "famous" person to do it?). Now that I think about it, it's funny to think about who gets "famous" and who doesn't, among the groups of truly talented people. John Prine would be relegated to the ranks of great songwriters no one has ever heard of, except that, well, nobody really does his stuff any better than he does his own; and he does it very, very well; consequently, he may not be a household name, but he's still very well known, and can fill a concert hall pretty much anywhere on earth.
Willie Nelson is of course about as famous as anyone, but he's another who, even if you'd never heard of him, you would have heard from him, because he's written so many hit songs done by other people.
John Hiatt's songs are probably more famous than he is (again partially thanks to Bonnie Raitt)
Led Zepplin is a great place to start for finding obscure songwriters, since they were notorious for ripping off other people's work w/o credit (makes it a little hard to search, but the info is out there).