A simple puzzle game, of which an inexpensive handheld electronic version was made by Tiger Electronics.
The game is played on a 5x5 grid of lights, which can be "on" or "off". The game starts with a random arrangement of lights on. The object of the game is to turn all the lights out. How do you do this? Touching any light toggles its setting, and also toggles the setting of the lights one square up, down, left, and right from the selected light. (If the touched light is on a border or corner, only toggle those lights which are on the board. The board does not wrap around.)
It sounds very simple, but it can be very frustrating. Once you get down to one or two lights on, it's not so easy to put them out.
I would put a whole bunch of ASCII art fun here to illustrate the concept, but there's a web-based implementation at http://www.websitetoolbox.net/lightsout.html. Enjoy!