The famous bass riff from Deee-Lite's "Groove Is In The Heart" is nicked from Herbie Hancock's "Bring Down The Birds".
http://www.youtube.com/v/1foayJLtUDc&rel=1
The oddest example of borrowing a riff/melody I know of is Right Said Fred's "I'm Too Sexy", which takes its instrumental riff from Jimi Hendrix' "Third Stone From The Sun", a very odd psychedelic track.
Verve used an Orchestral version of "The Last Time" by the Rolling Stones on "Bittersweet Symphony" but, having said that, the Stones nicked the song from "The Staple Singers" (a 1950's Gospel group).
I sense a new thread
PS. RZ, "Groove is in the Heart" is one of my all-time favourite songs, thanks for the Herbie Hancock info, I love the original now as well.
Another good piece of 'riff theft' from a great song to make another great song is taking "Toots and the Maytals' "- "54-46 Was My Number", increasing the BPM and coming up with "Double Trouble And Rebel MC" - "Street Tuff"
I think Mylene Klaas has sampled Beethoven on some of her albums as well.