Blacksn0w is an application for jailbreaking and unlocking the Apple iPhone. It unlocks the device to allow for the installation of third party applications and modifications to the device’s operating system. It also enables the use of different carriers and the ability to create custom ringtones. Blacksn0w is currently the only jailbreaking solution for the iPhone 3G, 3GS and iPhone 4. It can be installed either through a web browser or through the Blacksn0w application in the Cydia store. It is available for both Windows and Mac operating systems.
Discontinued The last update appears to be 4 years old: http://blog.iphone-dev.org/tagged/PwnageTool