Redsnow allows you to easily jailbreak your unlocked iPhone by slipping Cydia and overall making it much more capable than what Apple wanted your iPhone to be. For those who do not know, Redsnow also allows users to install Cydia which is a very popular third party application installer. It allows users to install myriad of tweaks, homebrew apps and so on.
Get it from here if you run Windows or from here if you run a Mac. Unfortunately, there is no Linux version of it. So, if you are a Linux user, you will either have to dual boot or use VMware to run XP inside Linux.
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