The most famous is probably Mr. Monroe's Blue Moon of Kentucky, which was Elvis' first big hit and which he (Elvis) changed from waltz time to straight time.

The story is that Mr. Monroe was asked how he felt about it, everyone thinking that he'd be outraged, and that all he said was, "Those were powerful royalties. Powerful."

Also: Haggard and Jones on Willie's "Yesterday's Wine." (I say that's folk music, that's who says it's folk music.) Emmylou on Townes Van Zant's Poncho and Lefty. Mangsen, Hills, and Herdman on Nancy White's Daughters of Feminists...

And anyone other than Dylan who ever sang any Dylan song.