[HD2 Android Q&A, Help & Troubleshooting and Genera] thatruth132:
FYI These codes have been obtained by decoding various .apk files present in the phone firmware.

*#*#4636#*#*

This code can be used to get some interesting information about your phone and battery. It shows following 4 menus on screen:

Phone information

Battery information

Battery history

Usage statistics

*#*#7780#*#*

This code can be used for a factory data reset. It'll remove following things:

Google account settings stored in your phone

System and application data and settings

Downloaded applications

It'll NOT remove:

Current system software and bundled applications

SD card files e.g. photos, music files, etc.

PS: Once you give this code, you get a prompt screen asking you to click on "Reset phone" button. So you get a chance to cancel your operation.

*2767*3855#

Think before you give this code. This code is used for factory format. It'll remove all files and settings including the internal memory storage. It'll also reinstall the phone firmware.

PS: Once you give this code, there is no way to cancel the operation unless you remove the battery from the phone. So think twice before giving this code.

*#*#34971539#*#*

This code is used to get information about phone camera. It shows following 4 menus:

Update camera firmware in image (Don't try this option)

Update camera firmware in SD card

Get camera firmware version

Get firmware update count

WARNING: Never use the first option otherwise your phone camera will stop working and you'll need to take your phone to service center to reinstall camera firmware.

*#*#7594#*#*

This one is my favorite one. This code can be used to change the "End Call / Power" button action in your phone. Be default, if you long press the button, it shows a screen asking you to select any option from Silent mode, Airplane mode and Power off.

You can change this action using this code. You can enable direct power off on this button so you don't need to waste your time in selecting the option.

*#*#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.

WLAN, GPS and Bluetooth Test Codes:

*#*#232339#*#* OR *#*#526#*#* OR *#*#528#*#* - WLAN 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

*#*#8255#*#*

This code can be used to launch GTalk Service Monitor.

Codes to get Firmware version information:

*#*#4986*2650468#*#* - PDA, Phone, H/W, RFCallDate

*#*#1234#*#* - PDA and Phone

*#*#1111#*#* - FTA SW Version

*#*#2222#*#* - FTA HW Version

*#*#44336#*#* - PDA, Phone, CSC, Build Time, Changelist number

Codes to launch various Factory Tests:

*#*#0283#*#* - Packet Loopback

*#*#0*#*#* - LCD test

*#*#0673#*#* OR *#*#0289#*#* - Melody test

*#*#0842#*#* - Device test (Vibration test and BackLight test)

*#*#2663#*#* - Touch screen version

*#*#2664#*#* - Touch screen test

*#*#0588#*#* - Proximity sensor test

*#*#3264#*#* - RAM version

NOTE: All above codes have been checked on Google Android phone Samsung Galaxy I7500 only but they should also work in other Google Android phones.