Ultrasn0w is a software application for unlocking iPhone and iPad devices. It enables users to unlock their devices using an unlock code. This app works with all baseband versions of iPhone, iPad, and iPod touch. It is essentially a jailbreak tool that allows users to access the root files of their devices and make modifications to the operating system. Ultrasn0w is an important tool for those who wish to unlock their devices for use with a different network provider. It can also be used to enable features such as tethering and video recording that are not available on devices locked to a specific network. Ultrasn0w is the first unlocking solution released for the iPhone and is compatible with all iPhone versions.
Discontinued Last update appears to be 4 years old: http://blog.iphone-dev.org/tagged/PwnageTool