Greenpois0n is a software application created by the Chronic Dev Team that is used to jailbreak iOS devices including the iPhone, iPod Touch, iPad, and Apple TV. It allows users to gain root access to their devices, allowing them to customize their device and install unofficial applications, tweaks, and themes. Jailbreaking enables users to bypass restrictions imposed by Apple and allows users to access the entire operating system, giving them full control over their device. Greenpois0n is a free and open-source application and is available for both Mac and Windows.
Discontinued Last update appears to be 4 years old: http://blog.iphone-dev.org/tagged/PwnageTool