GameBoy. Who can forget GameBoy? Playing games on the go. The first game system that was portable and ran on batteries (4 batteries, but still. It was portable! Even if you had to pay $40.00/month for batteries). Nintendo scored big with this, considering this was the first hand-held. This is what spawned PSP, Nintendo DS, and GameBoy Advance. It has the all time classic Tetris, it had the Pokemon games (Red, Blue, Yellow, Gold, Silver, Pinball, Trading Card Game, and Green (Japanese)), Super Mario Land 1-3, Bubble Bobble, and others.
The only problem I've found so far is that the GameBoy games need to be blown on CONSTANTLY before you play it, and the pins inside the GameBoy. They collect dust. Very bad. Other then that, it's a good system.
Prices range from $5.00 to $50.00 on eBay.
Pokemon Trading Card Game
Pokemon Green (Japan only)
Super Mario Land
Super Mario Land 2
Super Mario Land 3
The Legend of the River King
NBA Jam: Tournament Edition
Game & Watch Gallery
Mighty Morphin Power Rangers: The Movie
Ken Griffey Jr. Major League Baseball