My favorite recent discovery is that, if you are stopped first at a red light next to someone who is paying absolutely no attention to what's going on, and you pull forward a few inches to give the car behind you some room, odds are decent that the person next to you will sense your movement in their peripheral vision, assume the light is green, and start to go before they realize their mistake and slam on the brakes. The look on their faces when this happens is priceless.
I would never do this on purpose--with my luck they'd keep going through the intersection and take out a daycare van or something--but it sure is funny when it happens by accident.