Released only in Japan, this is a Game Boy Pocket with a feature not seen on any other Game Boy: a backlight. This Indiglo-style light makes the whole screen glow a healthy teal, and can be turned on and off during gameplay. Unfortunately, it also reduces battery life when enabled.
Nintendo has yet to release a Game Boy with backlighting in the U.S.1, while other backlit systems (namely the Sega Nomad, Game Gear, and Atari Lynx, among others) have enjoyed a modicum of success. For those people who want to play Game Boy in the dark, a variety of third-party light solutions have been developed. Some take their own batteries, while newer ones can be powered by the link cable port. The Nyko Worm Light is among the more popular of these "front-lights," and retails for around $10 US.
Not to be confused with Game Boy Lite, a version of the Game Boy with fewer calories.
1 Much of the buzz about the Game Boy Advance SP has mentioned that the SP's screen is front-lit, not backlit. Nevertheless, reviews about the SP's screen have been almost entirely positive. Thanks to yerricde for mentioning the SP to me. | 
