Albums that are primarily by one artist, that feature a "guest artist" for a track (or two)

although maybe this should get extra points since it's a greatest hits collection.

Beastie Boys cover of "Bennie and the Jets" with Biz Markie on lead vocals is only on "The Sounds Of Science" comp but I guess it sorta fits.

this was originally released as a three-"track" Biz flexi in issue #2 of Grand Royal magazine

Fleetwood Mac's Bare Trees closes with "Thoughts on a Grey Day," recited by one Mrs. Scarrott.

'nashville skyline'

xp I think technically this still answers the question unless "greatest hits" albums are somehow exempt.

I mean, I think "songs with a guest vocalist that still feature every member of the band playing on them" don't really fit the intent of the thread. But you're going to count Beastie Boys songs with other vocalists, you also want Song For Junior, Light My Fire, Dr Lee PhD, and - duh - The Biz Vs The Nuge, which is just Biz and a sample.

I meant 'or are not the artist' I.e. neither of the duettists are the main artist or band.

I know, I know...

No one's mentioned "My Curse" on Afghan Whigs' "Gentlemen?" Sung by Marcy Mays of Scrawl. There is also that song on New Order's "Get Ready" sung (at the beginning) by Billy Corgan.

More Clash, Allen Ginsberg on "Ghetto Defendant."