This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
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.
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.