I believe the Twitter API is one of the most widely used API's and would be a good resource. I really like their documentation, it makes integration a lot eas

I believe the Twitter API is one of the most widely used API's and would be a good resource. I really like their documentation, it makes integration a lot easier to have it well defined. Also a huge plus with most API's is a test/development environment. This depends on what the product/service is, but in some cases this is a huge issue.