I think the track “Que Pasa Contigo” featuring Ibrahim Ferrer of the Buena Vista Social Club on the Gorillaz album is kind of bizarre. Not really a duet though.
Nick Cave and Kylie Minogue on “Where the Wild Roses Go” is an interesting choice. But since it acheives the exact effect that Nick Cave was looking for (innocent beautiful girl murdered by dark brooding man), it probably works pretty well.
There is also the bizarre spectacle of Serge Gainsbourg performing a reggae version of “La Marseilles” (“Aux Armes Et Cetera”) with Bob Marley’s backup singers.