The JDK/JRE as mentioned in previous answers is located in /Library/Java/JavaVirtualMachines//Contents/Home/. The JRE is dir under this Home.
Interestingly browsers doesn't use files from this location for java applet plugin. The location used by browsers is /Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home/.
In fact if you run JRE installer, it updates the files in this location rather than JDK/JRE location.