Zeppelin III is great.
My favorite song from Zeppelin III:
http://www.youtube.com/watch?v=7c1KCCXD7bw
Led Zeppelin III was the first band that got me interested in alternate tunings, since the first alternate tuning I tried was DADGAD (I believe Friends is in Open E.) They brought Celtic music to Americans, much like the Beatles brought Indian music to the West.