1.Robyn sings backup vocals on Britney Spears' "Piece of Me." She recorded the original demo and parts of her vocals were kept for the final mix.
2.Dave Navarro and Flea played guitar and bass on Alanis Morrissette's "You Oughta Know."
3.Rashida Jones from Parks and Recreation sang on three songs on Maroon 5's debut album – "Secret," "Tangled," and "Not Coming Home."
4.That's Richard Marx going "allll niiiight" in the background on Lionel Richie's "All Night Long."
5.A young Cher sings the crescendo part on The Righteous Brothers' "You've Lost That Lovin' Feeling."
6.Ke$ha sings backing vocals on Paris Hilton's song "Nothing In This World," which was written by her frequent collaborator, Dr. Luke.
7.That's Lenny Kravitz singing background vocals in Madonna's "Justify My Love." He co-wrote the song.
8.Jack White sings on Electric Six's hit "Danger! High Voltage," though he was credited under the pseudonym John S. O'Leary.
9.A pre-fame Katy Perry sings at the end of P.O.D.'s "Goodbye For Now."
10.Prince sings and plays several instruments on Kate Bush's "Why Should I Love You?" from her album The Red Shoes.
11.That's Adam Duritz from Counting Crows warbling in the background of The Wallflowers' "6th Ave Heartache."
12.Blink-182's Travis Barker drums on Lil Wayne's "I Am Not A Human Being."
13.Long before she was on Married...WIth Children, Futurarama, or Sons of Anarchy, Katey Sagal sang back up on Olivia Newtown John's hit "Soul Kiss."
14.Dusty Springfield sings on Elton John's hit "The Bitch Is Back."
15.George Harrison plays guitar on Tom Petty's classic "I Won't Back Down."
16.Both LaToya and Janet Jackson sing the "na na na" part in their brother Michael's hit "P.Y.T. (Pretty Young Thing)."
17.Trent Reznor sings backup on Tori Amos' "Past the Mission" on Under the Pink.
18.Beth Orton sings the lady robot parts on Beck's "Get Real Paid"...
19....and that's Christina Ricci as the robot girl in Beck's "Hell Yes."
20.Elvis Costello sings the low part on the second verse of Squeeze's hit "Tempted." He also produced the album it's taken from, East Side Story.
21.Luther Vandross got his big break singing on David Bowie's hit "Young Americans" in 1975.
22.Blind Melon frontman Shannon Hoon sang backing vocals on Guns N' Roses' hit "Don't Cry."
23.Jimmy Page was one of the top session guitarists in England before forming Led Zeppelin. You can hear him solo on Donovan's "Mellow Yellow" among many other British rock hits from the late '60s.
