iUnlock is the first free and open source utility by the iPhone Dev team to unlock the SIM Card of Apple iPhone, allowing you to use any GSM based SIM card on your mobile device. More importantly, the software has a graphical user interface (GUI) making it easier for the common man to unlock the iPhone.

iUnlock, Free utility to unlock GSM Apple iPhone SIM Card

Pre-requisites include: Any version of iTunes older than 7.4 and iPhone firmware version 1.02 (Settings–>General–>About–>Version)

+ The device needs to be jailbroken first in order to upload files and install third party applications on the iPhone.
-If you are a Windows user, download and install iBrickr.
-Mac users can download iNdependence, a Coaoa based application for Mac OS X that provides an easy-to-use interface for jailbreak, activation, SSH installation, and ringtone/wallpaper/application installation on your iPhone.

+ Download iUnlock GUI (mirrors- here, here and here) and upload Unlock.app to the “/Applications/” directory on your iPhone. More mirrors can be found at freeiphoneunlock.com

* Replace the iUnlock.app with this file [mirror]

You will now see the iUnlock icon on your mobile device. Run it and wait for 20 minutes for the unlocking procedure to complete.