1. Facebook API
If you have not heard anything about Facebook's different APIs, you must have been living in a hole for the past few years. Facebook has become the de facto social integration tool. With over 500 million active users, Facebook has become an unstoppable force of content generation and user interaction. I have even heard some people say that Facebook is becoming the internet. The company has strategically infiltrated every corner of the internet with "Like" buttons and "Login with Facebook" buttons. This is not an API that any developer can continue to ignore. Seriously. It is "do or die".
2. GoWalla API
GoWalla.com's API is a location-based web service. It provides developers with a way to convert between latitude/longitude coordinates and nearby establishments. GoWalla really hit big once it was debuted at the SXSW conference in 2008. You give GoWalla a latitude, a longitude and a radius (in miles) and it returns a feast of nearby establishments in JSON. The reason why I like this API so much more than GoWalla's main competitor, foursquare.com, is the type of data GoWalla gives access to and the fact they use a REST approach.
- GoWalla API Documentation
- Get an API Key
- Test out the API (without writing any code!)
3. Google Maps API
You can integrate a map of anywhere into anything now thanks to Google Maps. Yes, there are mapping APIs available (Bing and MapQuest). There are drawbacks and benefits to each. I have the most experience with Google Maps due to a project I worked on recently. The project required a robust map and the supplying service to allow many thousands of hits a day without incurring a cost (initially, at least). Bing's terms of service stated that the number of requests per day were closely monitored and highly restricted. MapQuest's API (see Documentation) was not flexible or stable enough to handle the complicated requests we were making from our service.
Google Maps API came out a strong contender and never disappointed. Google offers some extremely useful functionality and the service was very reliable.