Nokia is being extra generous this month and giving away one free N-Gage code to everyone, per N-Gage Arena UserName.
To get this game code, all you have to do is, head over to N-GageWizard.com and fill in your details. Once you do that, you should get an Email with your free N-Gage Game code.
Note: There were a few issues with codes sent out in the beginning, but it appears to be worked out now. Regardless, if the code you get is 13 digits instead of 15, email them back to get the correct code.
Edit : Lame, they've changed the giveaway rules and limited it to residents of the UK only. If you're in the UK, you can still get your code. If you're anywhere else, y'all are outta luck.