Roy Harper is forever associated with Pink Floyd because of his memorable vocals on Have A Cigar from Wish You Were Here (1975). Below is a bit from Echoes FAQ:
Roy Harper is a "street singer" from England, popular in the 1970's. Waters didn't like the way he was singing Have a Cigar and Harper was in another studio at the time making an album. So, they brought Harper in, and had him sing it. Though Waters said that Harper did "very well," he also now feels it would have been better to keep the song within the band. Harper performed with the band at their 1975 Knebworth concert (which was the last performance of Dark Side of the Moon with Roger Waters). Upon discovering that his stage costume had been stolen, he threw a tantrum, ripping upholstry and breaking the windows of one of the tour vans. During this he cut his band badly - an incident which inspired the hotel-ravaging scene in The Wall
[Feb 27]. See also what Roy Harper has to say about it.
A bit more information, courtesy of Scott Lindsey:Harper is much more a song-writer than a musician, in the same way Bob Dylan is. He's been recording since the late '60's and has released about 18 or so albums, including the duo effort with Page, What Ever Happened to Jugula? which also featured Gilmour. Like Dylan, he doesn't have the greatest voice, but it is somewhat unique, albeit not as recognizable as Dylan's. Much of his released material is simply vocal with acoustic guitar. He's very British and at times very political -- very much the cynic. His music isn't for just anyone and for some is an accquired taste. What some find "pointed" others find "grating."
Harper also helped out David Gilmour on his first solo album, helping to write and sing Short and Sweet (and Gilmour has worked extensively on Harper's albums). He appears on Gilmour's live tour video.