The iPhone Dev Team releases redsn0w that jailbreaks all the iPhone and iPod Touch devices (exclude iPhone 3G S).
The jailbreaking tool is working at Mac OS X and Windows via Pwnage Tool and Quickpwn.
The latest redsn0w tool provides some strings attached like iPhone 3G users with yellowsn0w unlock should not use it until they release the ultrasn0w that contains the unlocked baseband.
This redsn0w tool doesn’t support for the iPhone 3G S.
You can get more guides and download here.