Java Decompiler (JD) is generally recommended as a good, well, Java Decompiler. JD-Eclipse is the Eclipse plugin for JD.
I had problems on several different machines
I use jadeclipse instead, because it can't work in 3.6/3.7 eclipse
Update site http://webobjects.mdimension.com/jadclipse/3.6/
Intallation http://5thcross.wordpress.com/2009/05/20/installing-jadclipse-in-eclipse/