FanTent has a neat collection of YouTube videos featuring rap songs covered by non-rap artists.
See Nina Gordon cover Straight Outta Compton by NWA (believe it or not, Neato covered this a while back!), The Gourds cover Gin and Juice by Snoop Dog, Gilbert and Sullivan's mashup of Baby Got Back by Sir Mix-A-Lot, and more!
Excellent! Link [um, yeah, NSFW lyrics and some images - whaddaya expect?] - Thanks FanTent!