Queen: David Bowie joining up for "Under Pressure". Steve Howe playing a relatively short (compared to Yes and Asia standards) flamenco guitar solo on "Innuendo", which has become a classic and one loads of people around the world try to learn and replicate (oddly appropriate given that Brian May asked Steve to do it because he couldn't manage it). Also, the half-minute operatic section on "'Bohemian Rhapsody" is arguably one of the first things people recall about Queen in spite of being very different to 99% of things the band did before or since.
On the Frank Zappa album Broadway the Hard Way, Sting comes on halfway through and performs "Murder by Numbers".
Mariah Carey doing back up vocals for Babyface on a track called "Every time I Close My Eyes"
Jamie Foxx's video for "Blame It" has scores of celeb appearances. Ashley Scott from Jericho, Ron Howard, Forest Whitaker, Jake Gyllenhaal, Samuel L. Jackson, Quincy Jones, Cedric the Entertainer, Morris Chestnut, Clifton Powell, Alex Thomas, DeRay Davis, Joe (R&B singer), Mos Def, Tatyana Ali, Jalen Rose, Bill Bellamy, Electrik Red, Dawn Richard, Keshia Knight Pulliam and LeToya.
In turn, Elton John plays piano, organ and sings backing vocals on "Whatever Gets You Thru The Night" by John Lennon, and John playing guitar and singing backing vocals on Elton's cover of The Beatles' "Lucy In The Sky With Diamonds" They made a bet that if both got to number one, they would appear in concert together, which came true in 1974, when Lennon appeared onstage at an Elton show in Madison Square Garden to sing both songs and "I Saw Her Standing There". It would prove to be Lennon's last public concert performance.
Blake Shelton in Cady Groves' "This Little Girl" music video. He plays a towtruck driver who tows Cady's car while she is still inside it.
Ian Knutson wrote less than 1/4 of "Religion Song (Put Away The Gun)" by Everything Else.
Sandy Denny's vocals on Led Zeppelin's "The Battle of Evermore", the only guest vocalist on any Zeppelin song. Played live the part was usually sung by John Paul Jones.