Have you ever been working on a project, tracing through code, only to get to a point where you're inside your jvm's core class code? Probably. And you've probably noticed that the core classes don't display all that prettily in eclipse, you'll get some polished-up bytecode:
Well it's pretty easy to attach some source... especially if you're using ubuntu and openjdk-6. Try this:
sudo apt-get install openjdk-6-source
Give it a minute to finish, then, in the previous window, hit the Attach Source button. In the ensuing dialog, select "Add File", navigate to /usr/lib/jvm/java-6-openjdk/src.zip
Hit OK, close the .class window, then ctrl+click on your core-class again:
Boom. I know I get tired of looking at javadoc in a browser, then back to eclipse... I like to look at the implementation details too!