Codes for samsung

*#06# - IMEI number
*#0*# - Enter the service menu on newer phones like Galaxy S III
*#*#4636#*#* - Phone information, usage statistics and battery
*#*#34971539#*#* - Detailed camera information
*#*#273282*255*663282*#*#* - Immediate backup of all media files
*#*#197328640#*#* - Enable test mode for service
*#*#232339#*#* - Wireless LAN tests
*#*#0842#*#* - Backlight/vibration test
*#*#2664#*#* - Test the touchscreen
*#*#1111#*#* - FTA software version (1234 in the same code will give PDA and firmware version)
*#12580*369# - Software and hardware info
*#9090# - Diagnostic configuration
*#872564# - USB logging control
*#9900# - System dump mode
*#301279# - HSDPA/HSUPA Control Menu
*#7465625# - View phone lock status
*#*#7780#*#* - Reset the /data partition to factory state
*2767*3855# - Format device to factory state (will delete everything on phone)
##7764726 - Hidden service menu for Motorola Droid

You can find more of these spread across the internet, and they're usually very handy to have, even if just to show off your geekiness to your social circle.
Update x1: More codes!
*#*#7594#*#* - Enable direct powering down of device once this code is entered
*#*#273283*255*663282*#*#* - Make a quick backup of all the media files on your Android device
*#*#232338#*#* - Shows Wi-Fi MAC address
*#*#1472365#*#* - Perform a quick GPS test
*#*#1575#*#* - For a more advanced GPS test
*#*#0283#*#* - Perform a packet loopback test
*#*#0*#*#* - Run an LCD display test
*#*#0289#*#* - Run Audio test
*#*#2663#*#* - Show device's touch-screen version
*#*#0588#*#* - Perform a proximity sensor test
*#*#3264#*#* - Show RAM version
*#*#232331#*#* - Run Bluetooth test
*#*#232337#*# - Show device's Bluetooth address
*#*#7262626#*#* - Perform a field test
*#*#8255#*#* - Monitor Google Talk service
*#*#4986*2650468#*#* - Show Phone, Hardware, PDA, RF Call Date firmware info
*#*#1234#*#* - Show PDA and Phone firmware info
*#*#2222#*#* - Show FTA Hardware version
*#*#44336#*#* - Show Build time and change list number
*#*#8351#*#* - Enable voice dialing log mode, dial *#*#8350#*#* to disable it
##778 (+call) - Show EPST menu
These codes are specific to HTC devices only:
*#*#3424#*#* - Run HTC function test program
*#*#4636#*#* - Show HTC info menu
##8626337# - Run VOCODER
##33284# - Perform field test
*#*#8255#*#* - Launch Google Talk service monitor
##3424# - Run diagnostic mode
##3282# - Show EPST menu
##786# - Reverse Logistics Support