Java Decompiler

DJ Decompiler is pretty good:

http://www.softpedia.com/get/Programming/Debuggers-Decompilers-Dissasemblers/DJ-Java-Decompiler.shtml
( I have bought it myself)

but I am told by Luciano to preprocess classes with this:

http://sourceforge.net/projects/jadretro/

especially when they are written with Java 5 (generics) syntax
see also:

http://stackoverflow.com/questions/31353/is-jad-the-best-java-decompiler

Very useful the Eclipse Jad Plugin, here the wiki

Actually it seems that JAD is outdated, and now you should use jdeclipse
(just choose the update site http://java.decompiler.free.fr/jd-eclipse/update )