Instrumental on their debut album. A small part of the same intro can be heard half a minute in the Sonata di chitarra e violino con il suo basso continuo by Giovanni Battista Granata (1620-1687). Another possibly influence on Led Zeppelin was the intro to Davy Graham's Cry Me A River ('63).
Led Zeppelin [acoustic guitar part at the beginning of their famous Stairway To Heaven; Spirit was part of the bill during Led Zeppelin's first American tour]
Eric Clapton [admits using the Stairway To Heaven riff for Let It Grow]
Far Corporation [as Stairway To heaven; hit UK]
Little Roger & The Goosebumps [as Gillegan's Island; sued by Led Zeppelin]
Frank Zappa [did Stairway To Heaven as a grim reminder to the day his whole equipment went up in flames during a gig in Lausanne (Switserland), a fire so dense it inspired Deep Purple to write Smoke On The Water; they were watching from the other side of the Lake of Geneva without even knowing it was a concert room burning]
Rolf Harris [hilarious Stairway]
Led Zeppelin also played Spirit's Fresh Garbage live (see there).