I know Club Nintendo has been around for a couple of years now but I haven't formally written about it and to be honest, not many people take advantage of all the cool stuff you get for free just for taking 2 minute surveys on games/systems you buy.
Club Nintendo has a bunch of different rewards and they release more as time goes on at infrequent intervals. They have rewards such as posters (which are all over the walls of my man-cave and look awesome), reversible pouches for your DSs (which I 100% recommend), Game & Watch games, heck they even have hand towels with Mario designs on them.
How do you get these rewards? It's very simple. First you sign up for an account on Club Nintendo (free of course). You can even add all the members of your family into one family account and share the coins you earn. Once you have signed up, you can register systems and games (usually only ones Nintendo had an involvement in) and earn coins. Inside each Nintendo published/developed game is a piece of paper that has a code on it. You type that code into Club Nintendo's page and it will give you a survey asking why you bought the game and then about a week or so later you can complete a survey about your thoughts on the game. Once you take the various surveys, you earn coins that you can redeem on the rewards I previously linked to. All the ways you can earn coins are listed on this page .
If you register a Nintendo system, it will add 30 days to your warranty which is on top of the boatload of coins you get for systems as well. Also, if you earn enough coins throughout the Club Nintendo year (which runs from July 1 - June 30), you will be eligible to win a Platinum (earn 600+ coins in a year) or Gold prize (earn 300-599 coins in a year).
So like I said, there's plenty of reasons to join Club Nintendo today and start earning some coins. 1) It's free and you get pretty neat free stuff as well, 2) it's very easy to earn coins and not time consuming in the least, 3) you can extend your warranty on your Nintendo systems which who knows, may come through in a pinch!