These codes are used only by technicians, So be careful with them and use it at your own risk. Press "Ctrl+D" for Future Use! :)

Basic Codes:
*#*#7780#*#*   - This code is used for factory restore setting.This will remove Google account setting and System and application data and settings.
*2767*3855#   -  This code is used for factory format, and will remove all files and settings including the internal memory storage. It will also re install the firmware.
*#*#4636#*#*   - This code show information about your phone and battery.
*#*#273283*255*663282*#*#*    - This code opens a File copy screen where you can backup your media files e.g. Images, Sound, Video and Voice memo.
*#*#197328640#*#*    -  This code can be used to enter into Service mode. You can run various tests and change settings in the service mode.
*#*#7594#*#*   -  This code enable your "End call / Power" button into direct power off button without asking for selecting any option(silent mode, aero plane and power-off).
*#*#8255#*#*  -  This code can be used to launch G Talk Service Monitor.
*#*#34971539#*#*    -  This code is used to get camera information. Please avoid update camera firmware option.

W-LAN, GPS and Bluetooth Test Codes:
*#*#232339#*#* OR *#*#526#*#* OR *#*#528#*#*   -  W-LAN test (Use “Menu” button to start various tests).
*#*#232338#*#*    -  Shows WiFi MAC address.
*#*#1472365#*#*    -  GPS test.
*#*#1575#*#*    -  Another GPS test.
*#*#232331#*#*   -  Bluetooth test.
*#*#232337#*#    -  Shows Bluetooth device address.

Codes to launch various Factory Tests:
*#*#0842#*#*   - Device test (Vibration test and BackLight test).
*#*#0588#*#*    - Proximity sensor test.
*#*#0*#*#*    -  LCD test.
*#*#2664#*#*   -  Touch screen test.
*#*#2663#*#*    -  Touch screen version.
*#*#0283#*#*   -  Packet Loopback.
*#*#0673#*#* OR *#*#0289#*#*    -  Melody test.
*#*#3264#*#*    -  RAM version.

Code for firmware version information:
 *#*#1111#*#*   -  FTA SW Version.
*#*#2222#*#*   - FTA HW Version.
*#*#44336#*#* - PDA, Phone, CSC, Build Time, Changelist number.
*#*#4986*2650468#*#*   - PDA, Phone, H/W, RFCallDate.
*#*#1234#*#*  - PDA and Phone.