JDocs offers easy access to Javadocs for many widely used APIs, including the JDK, XML APIs such as Xalan, Xerces, JDom, Castor, and JiBX, frameworks such as Struts, utility APIs such as the Apache Commons APIs, and many others. Currently, about 50 APIs are covered on JDocs, but there are plans of extending this number to 100 or even 150 soon.
Best of all, JDocs does not only include the Javadocs themselves. It also offers a way for users to comment on individual classes. Assuming that people actually start to contribute, this feature could prove to be very useful, similar to the PHP documentation, which already follows this model.