JD is a replacement for the popular JAD Java Decompiler. Unfortunately, JAD is no longer maintained and doen’t support Java 5 and 6 language features.
JD can be used as a library, as a standalone GUI application or as an Eclipse IDE plugin.
The standalone GUI application doesn’t require an installation. It doesn’t even require Java to be installed (written in C++)!
The GUI is very fast and intuitive. It is available for windows, linux and osx.
More information can be found at the JD website.