I like finding out someone guested on someone's song - typically a more well known song. Did you know John Sebastian plays harmonica on the Doors Roadhouse Blues?
Shuggie Otis played bass guitar on Frank Zappa's original recording of "Peaches En Regalia" on "Hot Rats'.
Huey Lewis plays harmonica on two tracks from Phil Lynott's Solo in Soho album while Bob Benberg (aka Bob Seibenberg or Bob C Benberg) from Supertramp plays percussion on one.
Looking For Another Pure Love on Stevie Wonder's Talking Book Album features the guitar of Jeff Beck.
Joe Morello & Gene Wright (from Dave Brubeck's group) played on Simon & Garfunkel's '59th Street Bridge Song (Feelin' Groovy)'.
Roy Bittan played piano on Dire Straits "Skateaway" In the more obscure department, Dr. John played piano on some of sax man Hank Crawford's albums for the Fantasy label.
Jim James from My Morning Jacket offers backing vocals on Bright Eyes' "At The Bottom of Everything" and on Dr Dog's "Shame, Shame" James Taylor plays banjo on Neil Young's Harvest
Not just "Skateaway," but the whole damn album (Making Movies)! During the sessions, Roy was asked by Knopfler to join Dire Straits, but he turned down the offer, electing to stay with Springsteen's E Street Band!