I’m designing a JavaScript API and am looking for outstanding examples. Please point out libraries whose API you like, one answer per library. If you don’t mind, a comment explaining why you think it’s good would be useful.
Also, please upvote existing answers if you agree that some library that’s already posted is good.
Update: I’m looking for APIs which do their job well (though that job could be niche), which are a pleasure to use, involved a good learning curve, had great documentation/test coverage, result in simple client code for common cases, or similar. I’m not necessarily looking for an API that does some inconceivably awesome thing like Cappuccino or JS/UIX.
The JavaScript library I use most is JQuery (http://jquery.com/). I like it because it hides lots of browser-specific issues, allows people that don’t know much about UI (such as myself) to do a decent job, and the syntax that results in using it is very clean and concise. Specifically, the API for selecting elements from the DOM is very clean. To get the element whose ID is foo, its simply $(“#foo”).