Gratuitous guest spots where the singer has to point it out so that you notice

perhaps more subtle, but The Knife 'you take my breath away': "i'm in the first row on your show.. in the first row of the First Floor Power show"

Billy Shears?

CMON JAMES!

"Slash!" - Lenny Kravitz, 'Always on the Run'

I have no idea if this really counts, since it's the guest artist mentioning it on the lead artist's song:

Oh Maria Maria
She fell in love in East L.A
To the sounds of the guitar, yeah, yeah
Played by Carlos Santana

Had to think of that Santana thing to, I hate that song and always found that bit sooo ridiculous, as Santana then proceeds to play a riff which is subjectively at a headache-inducing pitch, but also objectively really simple. Namechecking the player would suggest something far more spectacular, following it up with that riff seems to give him somewhat more credit than he deserves for it.

From one guest artist to another: "Joni Mitchell never lies"

Shanice: "Blow, Branford, blow."

Melle Mel on "I feel for you" - "Chakha Kahn, etc"

Was Billy Shears a guest of Sgt. Pepper's band, or a member?

Indeed, he was the founder, as I recall from the great Robert Stigwood film.

'Go' by Grimes and Blood Diamonds, whose sole contribution seems to be to say 'Blood Diamonds' in a low-key way near the end

“The Intro and the Outro” by the Bonzo Dog Doo-Dah Band is this thing times a million.

Though if Wikipedia is to be believed when the singer goes “Eric Clapton, ukulele” it really is Eric Clapton playing the ukulele.

... and Adolf Hitler is definitely on vibes, I'd recognize his playing anywhere.