This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori 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.