Redsn0w 0.9.6b1 is a jailbreak tool that enables you to jailbreak the iPhone 3G and the second generation iPod Touch on iOS 4.1 or 4.0. This tool was already available for the Mac but the iPhone Dev Team has updated Redsn0w which makes it compatible with Windows. Windows users and Mac users can download Redsn0w from the Dev Team blog.

IF YOU USE THE ULTRASN0W UNLOCK, PLEASE WAIT FOR PWNAGETOOL TO SUPPORT 4.1.  DO NOT USE REDSN0W. That’s because to use redsn0w at 4.1, you need to already have updated to official 4.1 from Apple.  If you do that, you lose the ultrasn0w unlock (possibly forever).