Wikipedia has an exhaustive account of iPhone/iOS jailbreaking attempts and tools.
Some bloggers, such as Mic Wright at ElectricPic.com, are already making their wish lists of jailbroken applications, which need a custom installer such as Cydia. Wright listed five:
* Allowing Apple's FaceTime video chat to work on 3G cellular connections.* Support for 720p high definition video uploads, not just downloads.* Installing custom wallpapers and themes, changing iPhone icons, and so on, similar to the capabilities in Winterboard (iPhoneHeat.com has a Winterboard tutorial).* And an application to turn the iPhone 4 Wi-Fi adapter into a local hotspot, letting other devices attach to the iPhone and share its 3G connection to the Internet.* Wireless gaming controller to work with the Wii, Xbox 360 or PS3, exploiting the iPhone 4's new gyroscope.
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