Absolutely awesome song! Just beautiful, complex, moving guitar work. One of my top five favorite rock songs ever.
Interesting note many may not know about this song (quoted from Wikipedia, but I've seen this story on multiple sites):
George Harrison was reportedly the inspiration for "The Rain Song" when he made a comment to Led Zeppelin drummer John Bonham, about the fact that the group never wrote any ballads. In Light and Shade: Conversations with Jimmy Page, biographer Brad Tolinski quotes Page:
George [Harrison] was talking to Bonzo one evening and said, 'The problem with you guys is that you never do ballads.' I said, 'I'll give him a ballad,' and I wrote 'Rain Song,' which appears on Houses of the Holy. In fact, you'll notice I even quote 'Something' in the song's first two chords. — Jimmy Page