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Dubbed purplera1n, the iPhone 3G S jailbreak can be downloaded from this address.
Note that you’ll need a Windows PC (Windows 7 and Mac are not supported for the moment), an iPhone 3GS with 3.0 firmware and the latest version of iTunes in order to install the jailbreak.
Also, keep in mind that the jailbreak is a beta version, so be sure to back up all your data before running it.