Using the paddles from Pong to shuttle Pac-Man back and forth through his maze while dodging the aliens of Space Invaders. All while Donkey Kong showers the screen with barrels. This wonderfully schizoid mash-up of four of the most classic video games of all time is called Pacapong and it comes courtesy of a clever lad named Dick Poelen as his contribution to the Ludum Dare 58 video game jam.
I look at this screen and visions of my childhood erupt across my inner retina. This is the kind of thing that we used to dream up as kids: crossovers between video games. I think one idea was to have Pac-Man gobbling dots as he moved up Donkey Kong's girders. Kinda weird to see something like that actually come to pass.
Kotaku has more about Pacapong, including where you can play it (bear in mind that it's a two-player game just like Pong).