Every Sunday, Sam will highlight a new, rising social media application or campaign.

As more smartphones become equipped with GPS, geolocating social networks have become all the rage. Whereas early attempts, like Loopt, were based on GPS tracking, the hottest applications now are all based on a “check-in” model. Early adopters are flocking to two rivals — Gowalla and Foursquare, although the number of competitors keeps growing. Last week, popular review site Yelp added check-ins to its iPhone app. Foursquare, which recently won the Crunchie for Best Mobile Application, has taken the lead, now receiving over a million check-ins per week.

The check-in model provides an opt-in approach–your location is only broadcast to your friends if you choose to “check in” at a venue. If you’re studying at Blue State, you take out your Blackberry, iPhone, or Android, open Foursquare, and the GPS will show you nearby venues. Choose Blue State, write a message if you want, and check-in. There is a social gaming aspect to the applications, as well, turning your city into a game board. The user who checks-in to a venue most often is proclaimed the “mayor.” And with each check-in, you receive points, helping you climb up the city leaderboard. In Foursquare, users receive “badges” for accomplishments — whether a “Newbie” badge for checking into your first venue or a “Gym Rat” badge for checking into a venue tagged “gym” ten times.

These applications encourage exploration of your environment — users provide tips to help make the most of any venue, and the points system incentivizes going to venues you’ve never been to before. Advertisers have been excited by the business possibilities. Consumers across the world are actively telling Foursquare exactly where they are. If you check in to a liquor store, why not serve an ad for Bacardi? If I’m at the Yale Bowl, show me an ad for where to get dinner after. Foursquare may finally be the Holy Grail of local advertising–bars in New York and San Francisco have begun to offer free drinks or food to its mayor.

In New York or San Francisco, it’s difficult to become mayor of any venue because the application is so popular. In New Haven, however, you can feel like Lewis and Clark, as you help map the city by adding venues and checking in to locations for the first time. Maybe you can beat out Mark W. — the mayor of Toad’s.