This guide may void your warranty and I'm in no way responsible for any damage it does to anything or anyone.
First download and unzip the following two files:
Open the Odin .exe file.
Turn your phone off.
Hold volume down, home and power buttons and it will boot into download mode.
Plug phone into computer.
Press "PDA" in Odin and choose the CF-Root-SGS3-v6.4.tar file you extracted earlier.
Make sure the "Repartition" box is UNTICKED.
Press "Start" and wait for the process to complete. When it's done it'll reboot.
On your phone go to Settings > Applications, and make sure "Unknown Sources" is ticked.
In your internet browser on your phone go to http://ziz.im/sams3
Let the app install.
Now launch the app and choose "Unlock". Your phone will now unlock itself. Reboot and your phone is unlocked!
You may need to repeat this procedure if you update your phone's software.
N.B. If the phone asks for an unlock code after reboot, download the following file to your computer: http://www.androidrevolution.nl/downloader/download.php?file=SIM_Fix_v.0.1.zip
Transfer the zip file to your phone (don't unzip it).
Switch off your phone.
Hold down the Home button, Volume Up button and Power button.
Use the volume keys to navigate to Install Zip from SD card and use the Home button to select that option.
Navigate to where you downloaded the zip file earlier
Confirm that you want to install it.
Once it's installed, navigate to Reboot device and your phone should now be unlocked.