Recently I have tried JDcos and found very useful. It is a site where you can find javadoc APIs for most of the important java packages (like Java SE, Java EE, Apache Ant etc). Previously I used to bookmark in the browser most of these APIs links, now I can find all at a single place.
Moreover the GUI is much rich than the conventional frame based javadoc UI, especially the method navigator, source navigator etc.
It also provides viewing source code. Now I don't need to download and keep the source code just look at the implementation detail occasionally.
It also supports uploading APIs of projects which you actively participate, however I haven't tried that yet.
It is something worth trying. JDoc