Hidden menus and codes on cell phones are anything but new. Back in the days, even the old-school brick phones had these, allowing people who knew about them to access service options and software information. Today, a "secret" menu exists on most (if not all) popular smartphones, and to get to it, you just need to input the right code.
If there's any code on this thread that's not working for the device which it's specified for, let me know and I'll remove it from the list.
To use these codes, open up your dialer app and Input the code (just like as you would when dialing a phone number) for your smartphone according to It's brand. The Hidden menu will then open automatically.
Sony : *#*#7378423#*#*
Samsung : *#0*# AND *#7353#
Iphones : *3001#12345#*
Nokia Lumia : *#92702689# AND ##634#
Other Android Phones (E.G : Nexus 4) : *#*#4636#*#*
For All Android Phones (Shows IMEI) : *#06#
A little on Galaxy S5:
On VZW S5 you have to have root and do the following
/efs/carrier/HiddenMenu
Edit it from OFF to ON and reboot ;-)
Also some other codes, some may work on others but should be all Samsung:
*#1234# to check software version of phone.
*#12580*369# to check software and hardware information.
*#0228# Battery status (ADC, RSSI reading)
*#32489# Service mode
*#7780# Factory data reset galaxy S5
*2767*3855# Full factory reset (Don't dial it does not ask you to confirm unless you have problem)
*#0*# To check the various hardware components of your Samsung Galaxy S5
