ultrasn0w is a easy to use, multi-platform, multi-device jailbreaking and unlocking tool for the iPhone 3GS (latest iPhone), the iPhone 3G and also the iPod touch (1st generation and second generation). Currently it is available for Windows and Mac OS X.
This is the Windows version that supports the latest 3GS firmware from Apple.
For full details (which are often updated) visit the iphone DevTeam blog. http://blog.iphone-dev.org