I found this fun Instructable today that shows you how to lock a deadbolt without having the key. I can think of a few situations where this could come in handy:
Hmm.. I suppose those are the only situations I can think of where this would be useful…
Anyways, all you need is some tape (clear packing tape works great). Take about a yard of tape and fold it in half so that only a few inches on the end are still sticky. Securely attach the sticky end to the deadbolt knob, hold on to the other end, step outside, and shut the door. Pull the tape so the knob turns. Once the door is locked, tug on the tape so it breaks off so you don’t have a strip of tape hanging out of your door.
I tried it a few minutes ago and it was very easy to do and very effective.