Best free Java .class viewer?

Procyon is a new open source decompiler that already beats JD-GUI in most cases. It's written in Java and comes in a self-contained jar. It is actively developed by StackOverflow's own Mike Strobel.

Eclipse will allow you to view the bytecode for classes, if the source is unavailable (search for 'disassembled bytecodes').

It seems there is also a third-party plugin that uses asm here.

JAD is one of the best Java Decompiler today. This is one brilliant piece of software. Nevertheless, the last JDK supported by JAD 1.5.8 (Apr 14, 2001) is JDK 1.3.

DJ Java Decompiler, JadClipse, Cavaj and JarInspector are powered by Jad. The last version of Decafe Pro has been released on 2002-01-03.

These viewers can not display Java 5 sources.

So, I use JD-GUI : logic, I'm the author :)