Secret

List of all codes: http://www.askvg.com/google-android-hidden-secret-codes/

The above mentioned link contains list of several hidden secret codes which can be used in any Google Android mobile phone to access things which are not accessible by default.

You can enable/disable lots of hidden settings in the mobile phone with the help of these codes. These codes can also show many useful information about your phone.

Also check:

http://www.askvg.com/revealing-3-hidden-secret-modes-in-google-android-mobile-phones/

http://www.askvg.com/samsung-wave-gt-s8500-bada-os-hidden-secret-codes/

http://www.askvg.com/list-of-some-hidden-secret-service-codes-for-chinese-mobile-phones/