iPhone Dev Team's iPhone 4, 3G, and 3GS software unlock, now with added PwnApple. Compatible with iPhone 4 baseband 01.59.00 and iPhone 3G/3GS basebands 04.26.08, 05.11.07, 05.12.01, 05.13.04, and 06.15.00! Thanks to @sherif_hashim and @Oranav for contributing baseband crashes.
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You can install UltraSn0w 1.2.7 from Cydia following a jailbreak.