Apple Inc. faces a growing threat to its iPhone business, as renegade stores spring up online to sell unauthorized software for the device.

The developer behind some popular iPhone software on Friday plans to open a service called Cydia Store that could potentially sell hundreds of iPhone applications that are not available through Apple's official store.

Users must download special software that alters their iPhones before they can run these programs.

Another small company plans a store called Rock Your Phone for iPhone users who have not yet modified their devices to make it easier to download and buy unauthorized applications. A third start-up is building an online store that specializes in selling adult games for the iPhone.

The new stores take aim at one of the underpinnings of the iPhone's success: Apple's App Store.

The upstart sites can carry software programs that Apple's official store won't, since the company tightly controls the kinds of applications it allows.

Among the programs that Apple doesn't allow is a free one called Cycorder, which turns the iPhone into a camcorder. Another program, which costs $29, dubbed PdaNET lets people use their iPhones as laptop modems to connect to the Internet.

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