Ram Jam’s (Black Betty) Bill Bartlett also played guitar on The Lemon Piper’s Green Tambourine. Speaking of which, the dude is 80 years old now!
Eddie Van Halen on Nicolette Larson's "Can't Get Away From You" Neal Schon played on the first Betty Davis album.
Steven Tyler sings a backing vocal on The Left Banke's "My Friend Today" and "Dark is the Bark" (from their second LP).
Peter Buck played on Dream Academy's first LP. Not "Life in a Northern Town," though; he's on "The Party."
I met the Spinal Tap guys 8-9 years ago and brought a few things for them to sign. Christopher Guest signed this as Nigel (as he does most things he plays under that name).
According to his book, this song caused Tom Petty to break his hand on the dashboard of the car when he heard it on the radio. He had turned the chance to work with riff Mike Campbell brought him.
I recorded Tommy Bolin's band back in 1976 and had to interface with his keyboard player, Mark Stein. He had his own mixer which I took a stereo feed from. I had no idea at the time that he was a same guy who was responsible for Vanilla Fudge, whom I absolutely loved back in the 60s. Funny thing is that his playing kind of reminded me of VF but I had absolutely no idea why.
He sure broke his hand a lot, during the "Southern Accents" sessions he punched his fist thru a wall, breaking it into pieces
Eric Clapton (yes, him again!) plays dobro on "Jenny Fey", the final track on Canadian Corey Hart's first LP, First Offense (1983). "For some reason, John [Astley, producer] felt there was one song Eric Clapton would really like. So just out of the blue he sent him the song, and lo and behold Eric Clapton was on the phone saying he wanted to play on it and asking [if] I would mind if he did. “I didn’t.”
the Help Yourself album by Julian Lennon has songs co written by Glenn Tilbrook of Squeeze and Buchannan/Bell of the Blue Nile. Note: this record sounds more like a Nineties Pink Floyd , due to the involvement of Bob Erin and James Guthrie .
A number of folks have noted here Brian Jones sax playing on The Beatles "You Know My Name..." but he is also making noises and singing on "Yellow Submarine" along with Patty Boyd and Marianne Faithful.