ProGuard is a great Java bytecode obfuscator.  Its also free.  To get it working on OSX there are only a couple of things you need to do.  Open a Terminal (I prefer iTerm) and execute the following commands:

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cd $JAVA_HOME/lib
sudo ln -s ../../Classes/classes.jar rt.jar
sudo ln -s ../../Classes/jsse.jar .

Also, if you are going to be signing any JAR files, be sure to do that after running ProGuard.  Running the jarsigner on the JAR file before obfuscating it with ProGuard might cause a ClassNotFound exception when trying to run your applet or application.