As with the Google APIs, the Yahoo Developer Network APIs has a great deal of functionality. Some of the most popular APIs are the Search APIs, Flickr API, and Maps APIs. They also have an extensive documentation on how to implement the APIs and how to use them. Before you start, you will need to get a API key, but that is an easy process that you complete of the Yahoo Developer Network website.
The Yahoo Developer Network also has a great deal of tools for the programmers out their. One of the newest is Yahoo! Query Language (YQL), a expressive SQL-like language that lets you query, filter, and join data across Web services. Like all the documentation, it is very extensive and easy to understand. The only problem I see with the Yahoo Developer Network is that if you do not already have web development or programming experience, it can be very easy to get over your head.