Recently I had the need of "decompiling" some class files...! Well, I had used some time ago Jad, but it was time to integrate it with Eclipse.
I couldn't imagine how easy it is already. The only thing you need is jadclipse. Download the small jadclipse.jar and put it in your plugins directory.
Then restart eclipse and after opening check on Windows/Preferences that the *.class files are Associated with jadclipse.
Also don't forget under Java/JadClipse to enter the path of your jad.exe (if you don't have jad.exe in your PATH, you need to configure this explicit)
That's it! Now you can open you external jar library or some class files within your project with a double click on the *.class file!
This also is very useful while you are debugging, causing Eclipse to display you the correct source line of your external class.
And finally, for those that want to make troubles to "jadders" please Obfuscate your code!