Select developers have had access to the API since the company's last hackathon and have been using it to build applications that take advantage of the Foursquare's push notifications. The API will go into a public beta just a few days before the company's global hackathon on Saturday.
So what kind of uses will this API have? BetaBeat's [and former ReadWriteWeb writer] Adrianne Jeffries describes it as a "magical tool that will allow developers to build apps that can remind you to buy milk when you walk past the grocery store."
A more specific example would be 4sqtransit, a service that sends up-to-date public transit schedule information to people when they check into a transit stop. It works by pairing a Foursquare user's check-in with data about transit stops and schedules from another database. Depending on their location, it sends a text message to the user containing the next few departure times.
4sqtransit creator Matt McCormick explains in further detail:
When a user checks in on Foursquare, I receive a notification from the Foursquare Push API that the user has checked in, with details about which of my users checked in and where they checked in at. My service then matches this Foursquare user to the user in my database to determine which transit agency they use, which they specified when they signed up for my application. I then query that transit agency for the nearest transit stop, based on the GPS coordinates of the user's check in location from Foursquare, and calculate the distance from the user to the transit stop. If the stop is within 100 meters of the user's check in location, then I move forward and deliver the stop times, otherwise I ignore the check in. To deliver stop times, I again query the user's transit agency for the stop times in the next 2 hours and send this information to the user by text message, using Twilio.
Pretty neat. It should be interesting to see how this API is used once it's available to a wider group of developers. For samples and documentation, check out its listing on Github.