It's entirely standard for rap songs to use samples, but what about the other way around --rap music sampled for use in non-rap songs? I can think of two examples: Macy Gray's Do Something which samples OutKast's 1994 song Git Up, Git Out, and Mary J. Blige's Reminisce which sampled Pete Rock & C.L. Smooth's They Reminisce Over You (T.R.O.Y.). Are there other songs which have used this technique?
Depending on your definition of "non-rap", I know for a fact that artists such as Girl Talk (along with others on the Illegal Art label) sampled plenty of rap for songs that weren't necessarily rap. Venturing into left field, there was a project called Kids & Explosions which put out an album of entirely sample-based stuff that I really couldn't pin down a genre on (edits were way too crazy/abrasive for it to be called a "mashup").