Cascading two of the same type of stompbox into one another. Usually, an overdrive is routed into a second overdrive for an extra boost — a practice used by Stevie Ray Vaughan and many other players. But you could also stack a distortion pedal into an overdrive. Some manufacturers, such as Visual Sound, even make pedals that combine two independent dirt pedals into one chassis.
You could also stack two delay pedals to create complex rhythmic echo effects, two choruses for an extra lush sound, and so on.