Codes are always good companion for you. You must have knowledge of them. Obviously these codes are not really secret and don't do anything amazing but since people commonly refer to them as secret codes it seems fair for me to do the same.
Entering these codes on android devices should work regardless of the hardware platform.

IMEI

*#06#
Standard IMEI command if you are trying to find out what your Android devices IMEI number is, the use this code.

Testing Mode

*#*#4636#*#*
It opens Phone Setting that Displays the hidden testing menu for android which provides details on application usage, battery health and status, Wifi information and general network information.

Field Test

*#*#7262626#*#*
Not supported by all units, will display a list of options for you to test and there is an option to log the information as well.

PUK Code Unlock

**05***#
If you need to unlock you Android phone this command can be run from the emergency calling screen , you will of course need your PUK code which you came with your Sim card alternatively contact your service provider to obtain the code.

Dialer Log Enabled

*#*#8351#*#*
Records your last 20 calls and stores them on your phone, you can find the data in the following
/data/data/com.android.voicedialer/app_logdir

Dialer Log Disabled

*#*#8350#*#*
Disables the above mentioned call logging The two most useful things here is the ability to confirm that you Android device battery is still working properly and being able to force your Android to stay on 3G without falling back to EDGE or GPRS if the signal is better. If you have any Android secret codes and hacks please post them bellow.