@RenStarDeluxe
Billy & Mandy got a lotta mileage outta this; Scooby Doo, Numbah 3, Fred Flintstone, Yogi Bear & Boo Boo, (kind of) the Powerpuff Girls, and Chowder.
My favorite one's in the KND crossover, where Ed, Edd, n' Eddy randomly show up for like 10 seconds.