I suppose that is the best way to sire someone. 
Although that’s more of a duet than just singing backup.
Sandy Denny (Fairport Convention) sang on Led Zepplin’s “The Battle Of Evermore”.
Michael McDonald sang on Little Feat’s “Red Streamliner”. Linda Rondstadt sang on their live (Waiting For Columbus) version of “All That You Dream”.
Sting did some spoken word stuff on Miles Davis’ “You’re Under Arrest”.
Natalie Merchant sings back-up on several of Billy Bragg’s tracks on Mermaid Avenue Vol 1 & 2. She also takes the lead on one song per album.
Kate Pierson (B-52’s) did backup for REM in Shiny Happy People
Paulina Rubio and Julieta Venegas did backup for Coti’s “Nada fue un Error”. As singers they were far more famous, he was known as a composer.
They also sang backup on Elton John’s “Don’t Let the Sun Go Down on Me,” along with Bruce Johnston. This was before the Captain and Tennille became famous, but Toni Tennille and her husband Daryl Dragon (The Captain) were in demand as session musicians and singers and toured with the Beach Boys for a good length of time.
Cher started as a session singer in the early 60’s for Phil Spector. Some of the songs she sang backup vocals on were “You’ve Lost That Loving Feeling” by the Righteous Brothers, “Da Do Ron Ron” by the Crystals, and The Ronettes’ “Be My Baby.”
It’s cheating, but on this version of “He Thinks He’ll Keep Her” Mary Chapin Carpenter is backed by Emmylou Harris, Kathy Mattea, Trisha Yearwood, Suzy Bogguss, Patty Loveless and Pam Tillis.
It’s a Girl Country Singer Conspiracy! 
Olivia Newton John was an uncredited but prominent backup singer in John Denver’s “Fly Away.”
Darryl Hall sang backup vocals on INXS’ “Burn For You.”
Pepsi and Shirlie (British pop duo) were originally Wham!'s backup singers.
Mick Jagger sang backup vocals on Living Colour’s “Glamour Boys.”
Chaka Khan sang backup for Steve Winwood on “Higher Love.”
Kirsty MacColl sang background vox on Morrissey’s “Interesting Drug.”
Chrissie Hynde sang backup on INXS’ “Full Moon, Dirty Hearts.”
On Gary U.S. Bonds’ song “This Little Girl,” the musicians were some band from New Jersey called the E Street Band and a guy named Bruce Somethingorother (Stringbean? Greenjeans? I dunno, but they say he was bossy.) sang backup. 
These are kind of genre-y, but…
“Inside out” by Reveille has two versions, one with backing vocals by the lead singer from Cold.
“Bring me to Life” by Evanescence has backing vocals by the lead singer of 12 Stones.
“This Mess We’re In” by PJ Harvey borrowed Thom Yorke of Radiohead as did “El President” by Drugstore.
“Dice” by Finley Quaye & William Orbit has backing vocals by Beth Orton.
I didn’t read through the whole thread to see if this was mentioned, but Michael Stipe from R.E.M. sang backup for Indigo Girls on “Kid Fears”.
Paul McCartney did the whispering backup on Donovan’s Mellow Yellow.
Don Williams sang backup for Kathy Mattea on Love At the Five and Dime.
Marcy Levy would also gain a somewhat larger degree of fame under the name Marcella Detroit, singing lead on the Shakespear’s Sister track “Stay.”
Michael Stipe sure gets around - he also sings backup on Kristin Hersh’s “Your Ghost.”
A young Marc Bolan does backing vocals on ‘Desdemona’ by John’s Children - a great track from the Nuggets collection.
Listen again, that sounds nothing like McCartney. It was definitely Donovan.
OK, wait a minute… wasn’t Michael McDonald considered at one time a member of Steely Dan? I think he was during Pretzel Logic anyway…
And as usual Biffy beat me to the “Tina Turner on Frank Zappa Albums” response… The high-speed stuff on Montana is pretty amazing.
Not so well known, but great Texas artist Terry Allen, backing vocals by high profile friend David Byrne, on Human Remains, one of my favorite all time records.
Highlight: Byrne harmonizing/yelling on “Buck Nekkkkiddd, yeaahhhhh!!!”
David Bowie on Lou Reed’s Satellite of Love
Olivia Newton-John on James Reyne’s Hammerhead.
And Phil Collins in the background of much John Martyn