On the first pressing of Led Zeppelin III, around the centre was printed "Do what thou wilt shall be the whole of the law".
Apparently there's a tape of the guys from Black Sabbath jamming with Zeppelin (the tape's known as 'Black Zeppelin'). I hear there's only a couple in existence.
The song 'War Pigs' off of Sabbath's 'Paranoid' was originally titled 'Walpurgis' and had darker lyrics. The label didn't like it that evil apparently, and so the song's title and lyrics were changed (though the original lyrics are still easily found on google I'm sure).