By “superstars”, I mean singers that were solidly at the top of their game in their genre when the song was recorded.
Examples;
Willie Nelson, Johnny Cash, Waylon Jennings and Kris Kristofferson performed The Highway Men.
Merle Haggard, Waylon Jennings, Charlie Pride, and David Allen Coe did Greatest Country and Western Song.
What other songs like these can you think of?
(They donts hayve ta bey country and weystern soungs.)
There’s all those charity songs like *We Are The World * and Do They Know It’s Christmas.
Would The Traveling Wilbury’s works count? 
Sure: You’ve got Dylan, Petty, Harrison, Orbison - four undeniable stars - and one very grateful Jeff Lynne. 
How about that godawful “All For One” song from the Three Musketeers movie? It had Sting, Rod Stewart and Bryan Adams.
And for the indie kids, you’ve got Jenny Lewis’ cover of “Handle With Care,” featuring cameos from Ben Gibbard and Conor Oberst. Sure, not household names, but in that sphere, those guys are as big as you get.
Under Pressure with Queen and David Bowie?
The Concert for Bangla Desh.
The Last Waltz was an all-star tribute to The Band. For the final number, “I shall Be Released,” everyone joined in: The Band (Robbie Robertson, Rick Danko, Levon Helm, Richard Manual, and Garth Hudson), Bob Dylan, Neil Young, Eric Clapton, Muddy Waters, Joni Mitchell, Van Morrison, Paul Butterfield, Neil Diamond, Dr. John, Emmylou Harris, Ronnie Hawkins, Ringo Starr, and Ron Wood. Probably the record for a non-charity event.
The version of “Lady Marmalade” in Moulin Rouge had Christine Aguilera, Little Kim, Mya, and Pink, all big names at the time.
Anything by the Beatles.
“We Taking Over Remix” - Akon, R. Kelly and T-Pain are three of the biggest stars in R&B right now. Young Jeezy and Lil’ Kim rapping just completes the package.
Ringo Starr recorded a version of the song “Drift Away” on which he is joined by Tom Petty, Alanis Morissette, and Steven Tyler.
Justin Timberlake, Nelly Furtado and Timbaland on “Give It To Me.”
The Trio CD had Linda Ronstadt, Emmylou Harris, and Dolly Parton singing together.
Just about anything by Crosby, Stills, Nash & Young
The Three Tenors project had, IIRC, Luciano Pavoratti, Placido Domingo, and that other guy.
The soundtrack of Robin and the Seven Hoods had some numbers with Frank Sinatra, Dean Martin, and Bing Crosby. You’ve Gotta Have Style was one.
It’s a shame they didn’t cover “Total Eclipse of the Heart,” just so Ben could sing turn around, bright eyes to Conor. 
Whats Goin’ On had a host of singers, all of whom were at the top of their game (Britney, Christina, Nsync, BSB, Bono, Fred Durst, Darren Hayes, and a host of other famous ppl).
“The Devil Comes Back To Georgia” featured Charlie Daniels, Johnny Cash, and Travis Tritt.
Bernadette Peters, Ladysmith Black Mambazo, Paul Simon, Pete Seeger, amd Itzhak Perlman, et. al. on Put Down the Duckie
That’s What Friends Are For : Dionne Warwick, Elton John, Gladys Knight, Stevie Wonder
“Yeah” by Usher, Ludacris, and Lil John. Although, I’m not sure if what Lil John’s doing on that song could be considered “singing.”