The single best way to get a fussy baby or small child to go to sleep. The technique is simple: once they're laying down, take your fingertip and start at center of the bridge of the nose between the eyes, stroking the skin gently and not-too-slowly toward the tip. Then lather, rinse, repeat for a couple of minutes.
The action seems to combine a soothing touch with a motion which draws the eyes down and closed. You can try it on yourself if you like, but like tickling, it's really only effective on someone else. If you don't have a fussy baby or small child to put to sleep, try it on your significant other after bedtime instead. It works just as well on older "kids", although it tends to take a bit longer.