jdocs.com & javaref.com

JDocs is a comprehensive online resource for Java API documentation. All the javadocs for a variety of popular packages are loaded into our db-driven system, and users can contribute their own notes to virtually any class, field, method. In short, JDocs provides a knowledge base defined around the major Java api's themselves, so you can find the information you're looking for right where it should be... in the documentation!

Javaref.com is a really slick step forward in the aggregation and sharing of Java api documentation. It uses a lot of ajax for a responsive interface, and it employs a wiki-like mechanism for community editing. Here's the list of features from the Javaref.com homepage