Interactive Web Physics
This is more of a simulation than a game, but physics is useful in games so this is worth
having a tinker with. The original website is http://www.iwphys.org/
where you can find the authors, updates, etc.
NOTE: The applet opens a window for the application to run.
Further Information
There is a tutorial to read, and you can
download the JAR file to run it as a local application
allowing you to design your own physics problems.
Since this it GPL, you can also download the source which
is a bit on the large side (13M).
Index of Java Games