This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
ProGuard (on SourceForge) is a good open source one. There are also commercial options (like Allatori, DashO, Smokescreen) that may work better than any of the free ones. And no, ultimately obfuscators can not stop the decompilation.