Yahoo! announced yesterday the availability of their new Answers API. The API gives you the ability to get answers by questions, category and by user. Like many of the Yahoo! APIs data can be retrieved in XML and JSON but also serialized PHP which is a handy convenience for PHP developers.
Nice to see that they've embedded an interactive "Try It Now" form within the body of the manual reference so you can learn more about the API without writing code (and with Ajax-y UI so no page refresh needed). Flickr for a long time has had the well executed API explorer that lets you interactively test-drive API calls and now Virtual Earth has a good interactive SDK page as well.