If you have installed iOS 5 GM on your iPhone, this jailbreak guide will guide you on how to jailbreak it. If you are a windows user, the most popular jailbreak tool is snowbreeze / sn0wbreeze. The tool supports iPhone 3GS, iPhone 4 but does not supports iPhone 4s.
First download snow breeze from this link. Unzip the package and start the snow breeze app. Click on the blue arrow to proceed. Local the iOS 5 IPSW file that you downloaded for your iPhone earlier.
On the next screen, select ‘expert mode’ and click on blue arrow again. Select ‘Build IPSW’ followed by clicking the blue arrow. It will now create a pre-jailbroken package. Click on ‘no thanks. I will stick to the spinner’. After a while, it will tell you put your device in DFU mode. Follow the instructions on the screen.
Then, you will told to start iTunes to flash to the new firmware. Select your iPhone from the left sidebar in iTunes and click on ‘restore’ button while holding the shift button on your keyboard. Browse for the firmware created by snow breeze. It will now flash the jailbroken firmware to your iPhone.
However, since it is a tethered jailbreak and not an untethered one, you will have to jailbreak it every time you do a reboot. For that we will use an app called iBooty. You will find the iBooty directory on your desktop. If not, download it from here and follow the onscreen instructions. That is it, you have successfully jailbroken your iPhone.
Mac users can follow this tutorial