Access hidden info on your Android device with these secret codes

Generic Android secret codes

The below codes will work on most Android devices, subject to carrier restrictions./ © ANDROIDPIT

These codes are mostly universal, and should work on Android devices, no matter the manufacturer. There might still be carrier restrictions on certain codes though, so they're not all guaranteed to work.
 *#06# - IMEI (International Mobile Station Equipment Identity) number - essential in case your device is stolen and you want to block the thief from accessing your network
 *#*#7594#*#* - Make Power button turn off your device without displaying menu
*#*#4636#*#* - Display information about phone, battery, usage and WiFi
*#*#7780#*#* - Factory reset (deletes app data and apps, not firmware)
*2767*3855# - Complete phone wipe, including firmware (not to be used lightly!)
*#*#273283*255*663282*#*#* - Back up all your photos and videos
*#*#1472365#*#* - Test your GPS
*#*#1234#*#* - Display phone firmware and PDA info

You can access the above screen on most Android devices by entering *#*#4636#*#* on the dialpad. / © ANDROIDPIT

*#*#232338#*#* - Show WiFi Mac address
*#*#8255#*#* - Access GTalk Service Monitor
*#*#36245#*#* - Access Email debug info Email
*#*#225#*#* - Events calendar
*#*#426#*#* - Debug info for Google Play Service
*#*#759#*#* - Access Google Partner Setup

Samsung secret codes

There are plenty of codes that we tested on Samsung devices (that may also work on others). / © ANDROIDPIT

Many of the above codes will also work on Samsung devices, but the below codes were tested specifically on Samsung devices, and we can confirm that they should work on them (subject to carrier restrictions).
*#*#34971539#*#* - Show all camera information
*#*#197328640#*#* - Enable test mode for services
*#*#232339#*#* - Wireless LAN test
*#*#1575#*#* - Test your GPS (alternative to generic Android GPS test)
*#*#0*#*#* - Test your LCD display
*#*#4986*2650468#*#* - Display crucial phone information (PDA, Hardware, firmware etc.)
##778 (followed by the Call button) - EPST menu

HTC secret codes

HTC phones like the One M9 can use the generic Android secret codes listed above, but also has some of its own codes. / © ANDROIDPIT

*#*#3424#*#* - HTC function test Program
*#*# - HTC Info menu
*#*#8255#*#* - Launch GTalk Service Monitor
##3424# - Diagnostic mode
##3282# - EPST menu
##8626337# - VOCODER
##33284# - Field trial menu
##786# - Reverse Logistics Support
##7738# - Protocol Revision

The function test menu on HTC phones can be accessed by typing ##3282# on the dialpad. / © ANDROIDPIT

There's a good chance you'll never need to use these codes, but if you do then now you know where to find them.
Are there any secret codes that you find yourself using all the time? Let us know!