I think it'd be helpful to have our own list of what we consider to be useful Nokia codes. Please only post codes and their functions on this thread - and only ones that are useful in the real world, not huge lists copy-and-pasted from elsewhere.

Here's a few to get us started:

*#06# - Displays handset IMEI
*#0000# - Displays handset software version
*#7780# - Restores default settings
*#7370# - Restores handset contents to factory state (you will lose all your data!)