Foursquare today launched the ability to add categories and subcategories to places listed in its service. The web-and-mobile-based social networking service now offers the ability for retail shops, restaurants, parks and many other locations to be properly categorized, indexed and found.
It likely won't be long before the categories are searchable, shareable and the service and its users capable of making recommendations. What should also be interesting is how business will reach out to new prospects who may be leaders in different local categories. For instance, if I'm a bookstore, it would be great to see the folks who check-in the most at bookstores in my area. They're likely a good prospect.
Small businesses that are already using Foursquare for marketing should make sure they are properly categorized. Of course, your Mayor probably already beat you to it!