Saw these on the Darla Mack blog, some people probably already know them but for those that don't:

The codes aren't published officially by Nokia, anywhere I've seen or read, and yet they have been covered by many sites and still frequently asked for by users everywhere. A new year seemed like a good time to review these useful codes. Use these codes at your own risk.

* *#06# - IMEI (serial number).
Structure of the IMEI:XXXXXX (TAC) XX (FAC) XXXXXX (SNR) X (SP)
o TAC = Type approval code
o FAC = Final assembly code
o SNR = Serial number
o SP = Spare
* *#0000# - Phone model, Firmware version and date
* *#7370# or *#res0# - Full phone format, wipes everything clean. Default security code 12345 until changed by the user.
* *#7780# - Phone reset, clears settings without deleting most data. Default security code 12345 until changed by the user.
* *#7370925538# or *#res0wallet# - Deletes everything in the Wallet, in case of forgotten password.
* *#92702689# or *#war0anty# - Lifetime timer, doesn't work on some older S60 devices and with older Nokias activated additional menus which has since been disabled or removed from the consumer firmware, now requiring a special Field Test firmware.
* *#2820# or *#bta0# - Displays the device's Bluetooth MAC address.
* *#62209526# or *#mac0wlan# - Displays the device's WLAN MAC address.
* [Green]+[def3]+[*+] - Fail-safe reformat, use this as a last resort if the regular format codes do not work or if the phone won't completely boot normally.