If you own a Nintendo 3DS XL, then as of today (and until the 15th of this month) you can download one game for free, as long as you agree to register your console with Club Nintendo. The offer was announced back in November, but today is the first day codes for free games obtained this way can be redeemed, and it's probably best to get it done now before you forget all about it and it ends up being too late.
To get your free download code, you need to find the Club Nintendo card that came with your console (in the box with all the instruction manuals) and use the code to register your system at www.club-nintendo.co.uk. Then, you'll have to fill out a registration survey on the Club Nintendo site, and click on a promotional banner that pops up again (I'm sure it'll be fairly obvious). Double-check that the email address you've linked to your Club Nintendo account is correct (because that's how you receive your code), and then follow the instructions on the site to choose your game. You'll be emailed a code, and all you have to do is make sure you redeem it (through the eshop on your 3DS console) before the 15th.
The games available are:
Super Mario 3D Land
Nintendo presents: New Style Boutique
New Art Academy
Freakyforms Deluxe Your Creations, Alive!
Professor Layton and the Miracle Mask
Which will you choose?