The documentation on jquery.com is in the form of a wiki, which means that the content is editable by the public. The site includes the full jQuery API, tutorials, getting started guides, a plug-in repository, and more:

On jQuery.com, the API is available in two locations—the documentation section and the paginated API browser.

The documentation section of jQuery.com includes not only jQuery methods, but also all of the jQuery selector expressions:

Jörn Zaeferrer has put together a convenient tree-view browser of the jQuery API with a search feature and alphabetical or categorical sorting:

This API browser designed by Yehuda Katz is both beautiful and convenient. It also provides quick viewing of methods for a number of jQuery plug-ins:

Sam Collet keeps a master list of jQuery documentation, including downloadable versions and cheat sheets, on his blog: