Heart does a cover of BoE that is technically better than Led Zeppelin because they have the built in duet. But it does not compare to BoE once you add in Sandy Denny’s amazing vocals in that song (her other stuff’s not bad either, but this is superb.)
Here’s a couple from one of the greatest movie soundtracks known to humankind:
Tom Waits and Crystal Gayle - “Picking Up After You”
Tom Waits and Crystal Gayle - “One From The Heart”
No mention yet of Let’s Duet by Dewey Cox w/Darlene Madison.
10,000 Maniacs with Michael Stipe-“A Campfire Song”
Michael Stipe only sings a small part but I guess you could still call it a duet.
“Christmas TV” by Slow Club
“In my Sleep” by Austin Hartley Leonard and Kendall Jane Meade
Well, I never thought it weird at the time (angsty teen!) but I suspect that I may be the only living person who ever actually owned the album “Nancy & Lee” 
I loved that album! I think Lee actually produced and/or wrote several of Nancy’s solo hits too.
Ha ha I have all three of their albums, as well as at least one other Hazlewood duet album. Plus plenty of his other stuff… I doubt we’re the only ones either!
The whole first Nancy & Lee album probably fits for this thread.
Just to clarify, it’s Merrie Clayton on the original studio version.
I’ll add Trey Anastasio (and Phish) and Allison Krauss, “If I Could.” Hippie schmaltz, not yuppie schmaltz.
Robert Plant and Allison Krauss have already been mentioned, but my preference is for Gone,Gone,Gone (Done Moved On).
I also like Of Men and Monsters, with the pop tune Little Talks being a current guilty pleasure.
Lemmy of Motorhead and Kelly Johnson of Girlschool [and the massed bands billed as ‘HeadGirl’] - Please don’t touch.
Kim Gordon [and Sonic Youth] with Chuck D - Kool thing. Arguably not really a duet but female lead vocalist with a bit of helpful nattering from the sides.
Our Kylie and Our Nick - Where the wild roses grow. Sort of the modern Lee and Nancy.
OK, I must somehow have missed all the earlier references to “It Takes Two” by Marvin Gaye and Kim Weston, because there’s no way we could have gotten this far in the thread without that tune being mentioned… right?
The Battle of Evermore with Robert Plant and Sandy Denny. She’s not the greatest singer ever, but it’s still one hell of a song. (Bonus: It’s about Lord of the Rings!)
Hmmm… coud this be a whoosh? If I were to make a short list of greatest singers ever, Sandy Denny might well be on it.
Huh. I definitely did a text search to see if anyone mentioned this. silenus scooped me. 
You want a non-cheesy male/female duet? Then you want Tomorrow Wendy by Andy Prieboy with Johnette Napolitano (of Concrete Blonde).
And also I Remember by Damien Rice with Lisa Hannigan (another link to a live version - the original is great too, but this version will knock your socks off, possibly literally.)
Some other great duets:
9 Crimes by Damien Rice with Lisa Hannigan
Yankee Bayonet - The Decemberists
Young Folks - Peter Bjorn & John
You Don’t Know Me - Ben Folds with Regina Spektor
Gasoline and Matches - Buddy & Julie Miller
I See You, You See Me - The Magic Numbers
Private Idaho - The B-52’s
Good Things - Luke Steele & Katy Steele
Your Ghost - Kristin Hersh with Michael Stipe
Where The Wild Roses Grow - Nick Cave and Kylie Minogue
Kids - Kylie Minogue & Robbie Williams (not sure it passes the cheese test but it’s sure as hell fun)
Kid Rock and Sheryl Crow, “Picture.”
Almost anything by Lady Antebellum.