I also modified the moddle and build another web site at http://enjoy.phy.ntnu.edu.tw/
The physlet builder and other simulation builder (circuit builder, mindmap builder...etc)
are part of the editor in our moodle system. We also have SVG drawing system(However, it only work for IE now.)
So user in our moodle system can use editor to draw simple diagram, create simulations, mathematic equations ...
However, it was all implemented in Chinese. I did not have time to port it to different language version.
The simulation builder was original written in javascript only. I port it to php script several years ago because some functions was not available with javascript at that time. I did not release  php code because it might lead to security risk.  Now, it is possible to separate the code with AJAX and many javascript library and make it safe to distribute the code.  I will try to find time to re-write the code and make it available for download so user can implement it to other system. (It is on my TODO list because a friend of mine is writing a slovokia version of physlet book, and he want to include simulation builder in the CD distribution.)  I will post the new version when it is available. But I need to find time to do it. I will announce here when it is available.  :)