Ok the rules are that the two music artists must have some kind of track record on their own beforehand and that the collaboration was only a one off or for a short period of time only (e.g. so you couldn't choose Simon and Garfunkel)

What songs ever which fit the above criteria are your favs ?

I love Kids by Robbie Williams and Kylie Minogue and recently i loved the Taylor Swift and Def Leppard collaboration at the Crossroads, totally awesome

Loretta Lynn & Jack White - Portland Oregon . This is a fantastic song that doesn't sound like it would work on paper.

Mark Knopfler & Emmylou Harris - All the Roadrunning. Not a big fan of either, but they sound great together on this CD.

In the spirit of the season, I'll say the Pogues and Kirsty MacColl doing "Fairytale of New York."

And depending on how one defines "short period of time," I'd also add Louis Armstrong and Ella Fitzgerald's three albums.

The correct answer is:

Bob Dylan & Johnny Cash — "Girl from the North Country"

My fave's off the top of my head...

Bryan Adams/Tina Turner - It's Only Love
http://www.youtube.com/watch?v=IfUyUhewyEQ

George Michael/Aretha Franklin - I Knew You Were Waiting (for me)
http://www.youtube.com/watch?v=AuU0cJ_iW8w

Tom Petty/Stevie Nicks - Stop Draggin' My Heart Around
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Kip Winger/Fiona - Everything You Do (You're Sexing Me)
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Dream Theater + Queensryche performing the Who's Won't Get Fooled Again live.

Actual studio track?
Alice in Chains & Chris Cornell on Right Turn.

My top two favorite bands are Led Zeppelin and Bad Company, so naturally I love the two The Firm albums with Page and Rogers.

Jesus & Mary Chain with Hope Sandoval (of Mazzy Star) with Sometimes Always

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Blink182/Robert Smith on All of this. Very good song off of a very good album.

maybe it's the inner gay in me, but Donna Summer and Barbra Streisand's duet is one of the very best things either of them have done

Wha? How dare you forget the third lead vocalist on that song, the mighty Mark Arm of Mudhoney!
(Actually, he would be the FOURTH lead vocalist on that track, as both Jerry and Layne sing, with Chris in between the two, and Mark the last part before they all go off at the end....)

Does such a giant collaboration truly count as only a duet?

This is absolutely amazing - two of the greatest singers ever totally going for it!
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Rob Halford and Bruce Dickinson - The One You Love To Hate

Yep!! That's an awesome track!

My other picks are:

RUSH and Aimee Mann..."Time Stands Still"
Ozzy Osborne and Lita Ford..."Close My Eyes Forever"

NOFX & Kim Shattuck (The Muffs) ~~ Lori Meyers

Korn & Amy Lee ~~ Freak On A Leash

Marilyn Manson & The Sneaker Pimps ~~ Long Hard Road Out Of Hell

Nine Inch Nails & David Bowie ~~ I'm Afraid Of Americans

Moby & Gwen Stefani ~~ Southside

Sepultura & Pavarotti ~~ Roots Bloody Roots

"Under Pressure" David Bowie & Queen

"The One You Love to Hate" Halford & Dickinson

"Absolution" Tribuzy w/Michael Kiske

"Bohemian Rhapsody" (Cover) Montserrat Caballe & Bruce Dickinson

Robert Plant/Alison Krauss

Elvis Costello/Burt Bacharach

U2 and Mary J Blige singing One (can't figure out how to embed the video :/
http://www.youtube.com/watch?v=Iu72EZdVou4&fmt=18

Lee Greenwood, Nancy Sinatra. Some velvet morning

Luther Vandross, Mariah Carey. Endless love

Linda Ronstadt, Aaron Neville. Don't know much

Johnny Cash and Joe Strummer - Redemption Song

Chris Cornell and Eddie Vedder. Hunger Strike

Thought of a couple more:

"I Got You Babe" (Cover) by UB40 & Chrissy Hynde
"A Tout Le Monde" by Megadeth and the singer of Lacuna Coil.

And, I'm not sure if it's the criteria of the original post, but "Something Stupid" by Frank & Nancy Sinatra is a great song.

Like this:

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To be technical, the song is by a singular band, Temple of the Dog, not by a duet of artists.

I was going to include Temple Of the Dog originally considering that CD itself was a one off, and only lasted a short period of time as for the rules stated

But then I remembered that "the two artists must have some kind of track record beforehand". So, being that Temple Of The Dog came out before Pearl Jam's first album, Eddie Vedder didn't have any sort of track record at that time. Therefore I kept them omitted from my list.

Which is a shame, cause its a highly underrated album and extremely good from start to finish.