There are various secret menus hidden in the Android OS. Here are some of them (thanks to askvg.com):

Code	Description
`##checkin##`	Forces the system to check for updated apps or a pending system update (found that one here first)
`##info##`	Shows various informations about GSM network, CDMA, WLAN, battery, etc.; also allows you to change a few
`##rst0##`	Will do a FACTORY DATA RESET after a confirmation
`2767full#`	Will do a FACTORY FORMAT – :warning: WITHOUT CONFIRMATION :warning: Be careful!
`##34971539##`	Get information about camera (NEVER try to update the camera firmware!!)
`##7594##`	Lets you change turn on/off the Power off menu
`##createall663282##*`	opens a screen where you can copy your internal media files to SD
`##197328640##`	Service tests, you can also change some settings there
`##talk##`	Google Talk service status

VANITY Keyboard

Android Secret Menus