Redsn0w 0.8 (iPhone 3GS Jailbreak) | 11.9 Mb

redsn0w is a easy to use, multi-platform, multi-device jailbreaking and unlocking tool for the iPhone 2G (original iPhone), the iPhone 3G/3GS and also the iPod touch(1st generation and second generation). Currently it is available for Windows, Mac OS X and Linux.

This is the Windows version that supports the 3.0 firmware from Apple.
For full details (which are often updated) visit the iphone DevTeam blog.

Instructions:
To use redsn0w simply upgrade the device in iTunes to firmware version 3.0 and run redsn0w to activate and jailbreak the device!!

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