Cross platform Java Decompiler with Eclipse plugin

JD-GUI is a free for non-commercial standalone GUI for displaying the source of ".class" files giving you access to methods and fields etc. Whats best is that this is available for Mac, Linux, and Windows.

The stand alone app is just one of 3 parts in a suite of tools. The other two being an eclipse plugin called "JD-Eclipse" and "JD-Core" which is a library that can "be used to recover lost source code and explore the source of Java runtime libraries"