For the server System.getProperties will return an enumeration containing lots of information, including JVM and version.
For the client, if you're using the OBJECT tag, instead of the APPLET tag, you can specify the plugin version. The browser will prompt the user to download the correct one if they don't already have it. There is a jsp:plugin tag that will take care of writing the object tag for you.
It is the policy here that we all display our real names. At some point a moderator will ask you to change yours to fit this policy (if you don't beat them to it). http://www.coderanch.com/forums/user/edit