We're doing a hackathon based on awesome publicly available APIs, where we will start with a user need and behaviour and then see if there's one or several APIs we could use to create a service.
Naturally, we will need APIs with great public, social and local data. APIs that can form the basis of a service. We would like a list with detailed descriptions of functions and limitations of the best public APIs out there (including information about rate limiting and authentication procedures), both well known and unknown. Additionally, can you find examples of other services built on the APIs and also let us know if it's free or if it requires a license.
Foursquare – Lets you do all the things you could do on our mobile application and website. Your users can check in, view their history, see where their friends are, create tips and lists, search for and learn more about venues, and access specials and recommendations.