Atari have introduced keychain size old school arcade type games such as pong and centipede. There are 2 variaties to choose from, either the Centipede & Milipede version or the Asteroids version. Unfortunatly they really are a little on the large size to be classed as keychains, but still, it's possible to use them that way 
. Both unit's measure 1.5" x 2" x 1.5" and have a built in cable to connect to most TV sets. The device is powered by 3 AAA batteries.
More details can be found at Coolest-Gadgets.