Instrumental:
Guitarists Ralph Towner (acoustic) and John Abercrombie (mostly electric) recorded some great albums on ECM. Some pretty amazing stuff, actually.
Chick Corea (piano) and Gary Burton (vibes) have recorded several on ECM also. More really good ECM recordings.
John McLaughlin (guitar) and his (then) wife classical pianist Katia Labeque recorded a great album called Mediterranean Concerto. The first side is a nice symphonic piece with the London Symphony, but second side are duets between McLaughlin and Labeque. This is musicianship at an almost superhuman level.
Drummer, Bill Bruford (Yes, King Crimson) and keyboardist, Patrick Moraz (Yes, Moody Blues) did a very unique album called 'Music for Piano and Drums' (duh...) that is sort of a mix of jazz and 20th century classical. This is a good one.
Vocals:
Jon Anderson (Yes) and Vangelis (of Chariots of Fire soundtrack fame) did a series of collaboration albums. They were a mix of New Age (some say they helped found the genre), electronic and prog. They are of mixed success, but when it works, it is very good.
Peter Gabriel and Kate Bush "Don't Give Up" from Gabriel's 'So' album.
