What, you thought I was talking about Stairway today in my post? Nope. I was actually thinking about this song in particular. This song was originally written and recorded by Jake Holmes, and then was later covered by the Yardbirds and Led Zeppelin. Zeppelin changed the lyrics around and adapted the melody slightly, and didn't credit Holmes on the original release of the album. Holmes later (in 2010) sued Jimmy Page for copyright infringement, but the case was settled out of court. Both Holmes and Zeppelin own separate copyrights for their versions of the song.
This is also one of a couple Zeppelin songs where Jimmy Page uses a violin bow on his guitar, in order to achieve the wailing effects you hear in the middle of the song.