I thought it would be useful to have quick access to entire java source code and for those open source projects that you don't have the source for while developing our java based applications. I came across this awesome link while searching for java source code online and now I visit it almost everyday. You can browse fully cross-references java source code from Maven repository just like you do in your IDE. I found it very useful and recommend it to every Java developer.
NOTE: A JavaGyan reader "Kallol Das" pointed out that the link above seems to be dead from quite some time. www.kickjava.com is another alternative but that too has availability issues at times.