1. The original Gameboy in 1989;
2. The Gameboy Pocket in 1996, which slimmed down the system and replaced the small pea-green monochrome screen with a slightly bigger black-and-white screen;
3. The Gameboy Color in 1998, which finally introduced a color graphics screen that encouraged programmers to develop games that used it;
4. The Gameboy Advance in 2001, which now had 32-bit technology for developing games that would be on par with the quality of Super NES and early Playstation games, besides also adding two shoulder buttons and making the cartridges now half the size of previous Gameboy system cartridges; and finally...
5. The Gameboy Advance SP in 2003, which reduced the system's size to a small foldable case and now featured screen backlighting that could be toggled on or off, plus a built-in rechargeable battery.

Of course, there was also the Super Gameboy accessory made by Nintendo for playing Gameboy games on the TV screen through the Super NES, as there is a similar device for the Gamecube system for playing all Gameboy games, including those for the Gameboy Advance.

Again, what made the Gameboy such a seller with people who would otherwise buy games for home systems was the quality and quantity of games that have been made since it was first released.  However, as the NES system had Super Mario Bros. as the pack-in game that helped sell that system into many American homes, the Gameboy system had the perfect pack-in game that helped sell as many Gameboys as could fit into people's pockets and backpacks -- Alexei Pajitnov's classic PC game Tetris, which showed how good a game can be made that doesn't have to rely on flashy graphics or sound.  Not that it didn't hurt that Nintendo had brought forth portable versions of its popular NES (and later Super NES) games like Super Mario Bros. and The Legend Of Zelda, as also did Konami, Capcom, and Acclaim plus a host of other software supporters who since passed on.  Several years later, though, Nintendo would have another game that would sell Gameboys like crazy -- the Pokemon series.

But the Gameboy wasn't the only portable game system in town.  Several other companies would try their hand at competing against Nintendo's handheld unit, only to fail for one reason or another.  The following are the prime examples of previously vital competing systems that came to challenge Nintendo:

Atari Lynx -- which came out in late 1989, the same year as the original Gameboy.  Besides having the obvious advantage of having a backlit color screen, the Lynx also had the ability to adjust the screen and controls for both left- and right-handed players.  Unfortunately, the Lynx didn't have the support that Nintendo's Gameboy did in having games developed by third-party manufacturers, and what games it did have released by Atari itself had a hard time competing with the games for the Gameboy system.  Even though the Lynx did eventually have hot licenses like 1992's Batman Returns movie and a few Ninja Gaiden games from Tecmo, they weren't enough to save the Lynx when Atari decided to pull the plug on its life support in the mid-1990s.

NEC TurboExpress -- which came out a year or so after their TurboGrafx-16 home game system did here in America.  Like the Atari Lynx, this system would also have a backlit color screen, although it used a special LCD color screen to prevent graphics from blurring during gameplaying.  However, its main draw was that it could play the same card-shaped cartridge games that the TurboGrafx-16 system played, so owners of both systems could take their favorite games with them anywhere.  Sadly, what killed the TurboExpress was its asking price of $300 and that it wasn't as widely available and accessible to gamers as were Nintendo's or Sega's systems, handheld or otherwise.  Add to these problems the same lack of third-party software support and its very short battery power lifespan (which the Atari Lynx also had), and you can see why the TurboExpress didn't enjoy a lasting place in the videogame market.

Sega Game Gear -- which came out in 1991.  Like the previous two systems, it would also have a backlit color screen, and like the TurboExpress before it, players could buy an optional adapter for watching their favorite TV shows on their handheld system.  Unlike the Lynx and the TurboExpress, the Game Gear had the advantage of third-party software support which Sega had garnered for their Genesis system after the ridiculous failure of allowing any for the Master System, and also some of the games that appeared on the Gameboy would also be converted into Game Gear titles.  It also got translations of some of Sega's heavy-hitting Genesis games like the Sonic the Hedgehog and Streets of Rage series. The biggest surprise of this system, however, was that the Game Gear was actually a Master System modified for use as a portable -- a fact that was ignored until some independent companies not licensed by Sega had developed and released cartridge adapters for playing Master System games on the Game Gear.  This system managed to hang on and compete with the Gameboy for several years, up until when Sega decided to try replacing it with its next handheld system.

Sega Nomad -- which came out in the mid-1990s.  This successor to the Game Gear had the significant advantage of playing Sega Genesis games without the need for an adapter, and not only that, players could hook it up to a TV and use it as a very portable Genesis system.  By the time this system came out, however, the Genesis was pretty much on the wane as far as software support went, and as bold as this system was in trying to keep the Genesis alive, it would join that system in the discount bins of toy and department stores that still sold Sega gameware.

SNK Neo-Geo Pocket Color -- which came out around the turn of the millennium.  This company tried as hard as it could to market a game system that players might want to own in lieu of or in addition to the Gameboy systems, even with offering scaled-down versions of its ever popular Neo-Geo fighting games.  Unfortunately, it was weak in the area of having other games besides fighting games -- just a translation of Namco's Pac-Man and Sega's Sonic The Hedgehog was all else that people ever saw.  The system became nothing more than a blip on the radar and then was gone.

Ben Heckendorn's portables -- which, while it may seem unfair to include in a list of systems that came against the Gameboy only to crash and burn, still fail on the grounds that these are not commercial products but rather the creation of one's ingenuity with modifying previous game systems into portable versions that only one person in the world, Ben himself, would be able to play.  But as his website shows, Ben does put a lot of creative touches into his portable machines, including his constant revisions of the VCSp, which has now included a paddle knob control and a second player controller plug in addition to some stylistic touches that would make the portable system feel at home with the furnishings of the late 1970s to early 1980s.  Had somebody actually gone into the business of making portable 2600 machines in the same mass quantities as Gameboy systems and with similar quality, these would give Ben's homemade portables a run for the money.

Will Nintendo continue to dominate the handheld gaming market?  Who knows?  Right now, Nintendo's getting its successor to the Gameboy Advance, the Nintendo DS, ready for release sometime soon with games that will show off its dual-screen 3D-graphics features, plus some non-gaming features that may get some buyers interested.  Sony in the meantime is also preparing to enter into that market with its Playstation Portable, the PSp, and Nokia's N-Gage has already jumped in feet first with a handful of games and features like a built-in cellular phone and text messaging with its powerful portable.  It will be a battle for which company's system is going to be played the most while on the go, but hopefully the gamers will be the real winners.