RetroGmer is a Dashboard widget that allows you to play classic arcade games like Donkey Kong, Super Mario, Mario Adventure, Tic Tac Toe, Pac Man, Space Invaders, Phoenix, or 1945 right in your Dashboard.
Seems to need flash to works.
Hence, if you have not installed flash and don't want of this like me, you cannot use it...
Precise it at least in the description. (And if connection is required. I don't know as I was unable to use it)