Sweet funky instrumentals. There's the Commodore's "Machine Gun", Barry White's "Love's Theme". There's a whole bunch of almost vocal-free tracks, like Average White Band's "Pick Up the Pieces", Kool and the Gang's "Jungle Boogie", or any of the great but brief instrumentals on Public Enemy's "Nation of Millions" and "Fear of a Black Planet". Hell, you could fill an entire album with great funk instrumentals just from James Brown alone. But mellow funky instrumentals are a rarer breed.
Some of the best ones, off the top of my head: "T Plays It Cool" by Marvin Gaye, "Summer Madness" by Kool and the Gang, Maxwell's "Arroz Con Pollo". Hmm, I think there are some more I'm overlooking. Ideas?