In this video tutorial, you will see how to jailbreak your Apple iPhone 3G using QuickPwn (these steps will work for the iPhone, iPhone 3G, and the original iPod Touch). By the way, this program, which is free, will work on both Windows and MAC OS X.
All you need is iTunes 8.0, a supported iPhone or iPod Touch flashed with software 2.1 and the software quickpwn from http://blog.iphone-dev.org/
The steps are really easy and you should be up and running in less than ten minutes. If you want to go back to the original software, all you have to do is restore it through iTunes. Simple, huh?