I was listening some songs written by an American group called Spirit. So I had a search for other albums by them and during the search I listened to a snippet of a one song called "Taurus". It suddenly dawned on me after a couple of bars that it sounded very like "Stairway to Heaven" by Zepplin!? So a quick search of Wikipedia uncovered the following.

Over the years, a number of people have put forth the opinion that the song's introduction, and opening guitar arpeggios, bear a close resemblance to the 1968 instrumental "Taurus" by the group Spirit. Zeppelin opened for Spirit in an early American tour, leaving little doubt that Led Zeppelin had heard the Spirit song before Stairway to Heaven was written. In the liner notes to the 1996 reissue of Spirit's debut album, songwriter Randy California writes:

People always ask me why "Stairway to Heaven" sounds exactly like "Taurus", which was released two years earlier. I know Led Zeppelin also played "Fresh Garbage" in their live set. They opened up for us on their first American tour.