I've always thought it was cool that only one Led Zeppelin song features a singer from outside the band: "The Battle of Evermore" from IV (1971) is a duet sung by Robert Plant and Sandy Denny, the great folk-rock singer who had a prolific career as a solo artist and with Fairport Convention. The song was written quickly according to Page, who said that its composition began with the first time that he tried a mandolin. Moments like this one are the reason for the thinking that some musicians follow, where the belief is that the sound will lead the player, not the other way around. It certainly worked in this case. It's a rich, dramatic cut that combines the blues with Tolkien (and other folk-tale elements), and without the use of electric guitars or drums is not out of place on an album that includes "When the Levee Breaks" (for example). A lot of the credit has to go to Plant and Denny, who create an other-worldly atmosphere with their vocals. Check it out: http://www.youtube.com/watch?v=uTuzVZsKAhY