This song wasn't actually written by Led Zeppelin, although their version is certainly the most famous. It appears as the third song on Physical Graffiti and is the longest song in the Led Zeppelin studio catalog.
(due to the Youtube restrictions, the video cuts a bit off the end of the song)
The song is a folk/blues standard and the lyrics are based on an old gospel tune called "Jesus Make Up My Dying Bed." The music here, however, is all original, featuring Jimmy Page on slide guitar in Open A tuning, and the writing credits list all four band members.
Here's a live version from 1975 that can be found on Led Zeppelin DVD:
Bob Dylan recorded a radically different version of the song on his debut album, but good luck finding any Dylan on Youtube.
The song was featured in the film It Might Get Loud with Jimmy Page teaching the tune to Jack White and The Edge.
Unfortunately the video is not on Youtube either but the audio is, and it's very interesting to hear such different approaches to slide guitar from three different generations of guitarists.
Sorry for the lack of updates recently, by the way, but the semester just started.