Normally we'd give a brand new site a break for not immediately having an API. When it comes to the rabidly popular FourSquare, which launched in March, developers have been clamoring to build on top of the service. So, today's launch of the FourSquare API (our FourSquare API profile) is bound to be met with extreme enthusiasm.
Your next geo app is going to be easier to build. Instead of running your own database and collecting geographic data yourself, you can build on top of TownMe and its newly-launched read/write Geo API (tech details at our TownMe API profile).