Secret Codes That Unlock Hidden Features on Your Phone.
The USSD protocol allows you to access hidden features you didn't know about right from your smartphone's dialer.
But there is some trickiness you'll need to know about.

During my time in a secret government agency called Girly Boy Squad we were trained in several different ways to diagnose and test the hardware and software on Android. Since I was the top Girly Boy in the squad it was my job to know these secret codes that would get you in and out of hidden menus within Android.

A point to be noted here is that some of the codes are a bit extreme.
Some of them wipe your entire smartphone and factory reset it so be cautious while inputting them.
Read the description and then go on a code entering spree.
That being said let's start with some great Android secret codes.

Windows keyboard shortcuts overview. Use shortcut keys as an alternative to the mouse when working in Windows. You can open close and navigate the Start menu desktop menus dialog boxes and Web pages using keyboard shortcuts.

Shortcut keys ABCs.
Shortcut keys help provide an easier and usually quicker method of navigating and executing commands in computer software programs.
Shortcut keys are commonly accessed by using the Alt key (on IBM compatible computers) Command key (on Apple computers)