For owners of smartphones (iphone, androids, etc.) foursquare is a fun little application to download and use on their phones. It's a location based application that lets users "check-in" at whatever location they may be at. Check-ins are shared with other foursquare users, either all users or a smaller designated circle of friends. For instance, when I come to work I check-in at my building - Armsby Building - as do some of my foursquare using colleagues. And we've created a friendly competition out of this resulting in me being the "mayor" of Armsby (i.e. I've checked-in most often during a 60 day time period).
But foursquare can be, and is, more than a fun game. Businesses can use foursquare to their advantage as a marketing tool. At the most basic level, it can be considered "word of mouth" advertising. Think of the power in having a foursquare user check-in at your business or market on a regular basis and that being shared with all of their friends! Beyond that though, you can use foursquare to offer specials to your foursquare-using customers. You can create a mayor's special, loyalty special, and newbie special, among others. Think of it as electronic couponing - rewarding those who visit you.
Analysis of foursquare data is also available - a must for determining value in your foursquare use. You can keep track of the number of check-ins, what time of day check-ins occur, as well as age and gender breakdown - all for a variety of timeperiods.
Hopefully you're intrigued. If so, check out this article from Mashable.com about how to get started with foursquare marketing. Foursquare itself also provides lots of assistance and information for businesses on using the application.