If you're using Java < 1.7 on OS X and you need to use JSObject for connecting an applet to JavaScript, you can find it in in $JAVA_HOME/jre/lib/plugin.jar.
The "plugin.jar" archive doesn't seem to exist on Java 1.7 for OS X (as packaged by Oracle). There is a jfxrt.jar that seems to contain JSObject, but that unfortunately means that you can't use the same plugin.jar that you can on almost any other JDK, including 7u5 for Linux, which still has plugin.jar built right in.
In particular, this gets irritating if you're trying to use it as a compilation dependency for a build tool such as Maven, which you could otherwise do with a JAVA_HOME-relative path.
Plugin.jar is available on Java 7u6 for OS X, now available.