I’ve used JAD for years, but it has been abandoned and doesn’t support the newer versions of Java well. If you still want to use JAD, soemone was kind enough to set up a new mirror for the last version: http://www.varaneckas.com/jad.
Fortunately, there’s an even better alternative, JD: http://java.decompiler.free.fr/. JD is available as either a standalone UI or a plugin for Eclipse (which is a bummer since I only use Intellij). This simple little tool has been pretty useful, but I still wish the Java world had something like the Lutz Reflector.