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Nintendo has owned the portable video-game market — first with the Game Boy, now with the DS — for so long that most of us gave up hope of ever seeing a viable competitor.

But it's become impossible to ignore the new kid on the block: Apple's iPhone, whose game library has quickly overwhelmed Nintendo's.

There are now more than 9,000 iPhone games available at Apple's App Store. The quality varies widely, and the market is so flooded that it's hard to sort the good from the bad.

But since the prices are low, you can buy 10 or so games for the price of one Nintendo cartridge. None of them will have the depth of, say, Rockstar's "Grand Theft Auto: Chinatown Wars," but they may be better suited for on-the-go action.

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My main problem with the iPhone is that its touch-screen controls aren't as precise as those on dedicated game machines.

I'm not about to replace my DS (or my PlayStation Portable, for that matter), but I do enjoy wading through the App Store's offerings. Here are some recent standouts.

—"Zen Bound" (Chillingo, $4.99): The best iPhone games are those that seem like they couldn't have worked on any other device.

Each level of this gem shows a 3D sculpture with a string attached, and the idea is to rotate the object with your finger until most of its surface is wrapped by the string.

Never has a game been more appropriately named; the challenging yet stress-free "Zen Bound" creates an eerie sense of calm. Four stars out of four.

—"Flight Control" (Firemint, $2.99): The most popular game at the App Store is as nerve-racking as "Zen Bound" is relaxing.

You use your finger to guide aircraft safely onto runways, but it isn't long before the skies are filled with planes waiting for you to bring them home.

A crash is inevitable — but that only makes you want to try again to get a better score. Three stars.

—"The Oregon Trail" (Gameloft, $5.99): Sure, pioneer travel was awful, but this revamp of an '80s classic brings it to life with vivid animation and wacky humor. Try not to let your kids get abducted by eagles. Three stars.

—"Magnificent Gizmos & Gadgets" (Publisher X, $1.99): This nifty physics-based puzzler will appeal to anyone who ever thought it would be fun to build a Rube Goldberg machine. Two-and-a-half stars.

—"WordFu" (ngmoco, 99 cents): Roll the lettered dice and find as many words as you can. It's a lighter, faster version of Boggle, and not quite as satisfying. Two stars.

—"Metal Gear Solid Touch" (Konami, $7.99): Konami turns its epic espionage saga into a drab shooting gallery. A blatant case of trying to squeeze a high-profile franchise onto the wrong platform. One star.