Catman
A few years ago, a friend of mine was visiting me at my old apartment. We found an old slide projector and decided to have a slide show with my parents' old pictures from the '70s and '80s. The best surface turned out to be the bare concrete wall on the balcony, so we set it up out there. My apartment overlooks a parking lot which is dark at night, so we had the idea to shine the projector on the ground. It worked better with some images than others. Actually, you can see what it looked like here:
www.angelfire.com/apes/digitalgrotto/031slide.html

By far the best slide was of a cat Jack-o-lantern my dad carved. We flashed it all over the parking lot and said we were using the Cat-signal to summon Cat-man. Eventually that night, we thwarted two crimes:

1) Two shady kids walking through the parking lot were trying to open all the car doors. When they saw the Cat-Signal they knew what was in store and ran to their homes, where they repented.
2) A girl went into a dark corner obviously to relieve herself. We didn't interrupt her until she was done, when she suddenly saw the Cat-signal, and beat a hasty retreat.
(racetraitor has an alibi for, Fri 12 Oct 2007, 11:30, Reply)