JODE (on SourceForge) is a good one. It doesn't understand most of the class file features introduced in Java 5, though. Keep in mind that it may be illegal to decompile source code you haven't written yourself.
Ping & DNS - updated with new look and Ping home screen widget
DJ uses JAD internally. It's free for most uses, except that you can't include it in your own software release. If that's not free enough, well JODE is licenced under the LGPL, so that should work for you. You may get other suggestions here.