Sync and Destroy 1: The Cookie Jar
Here is a demo which is a jar which hosts embedded demos which are loaded via URLClassLoader looped back to a builtin webserver, also hosting the article that explains it all... Total download size? 115k! (By the time you read this maybe more, but then it'll have more features :)
I had so much fun today with this, it shouldn't be legal!
The code is on http://java.net/projects/webservlet.
The next step is to try a nice splash screen, or interactive startup page, and/or login panel, to keep the user distracted while loading a larger application in the background (using URLClassLoader).