I am the founder and major maintainer of Ultrastudio.org.
Ultrastudio.org is an educational site built along the lines of “encyclopedia that anyone can edit” with content interactivity enhancements. Next to educational articles it also shows demos in the form of Java applets. These applets are written and contributed by community, same as the rest of content. The site is free to use for anyone, and most of the contents are under various Free licenses. All applets on the site are so called "unsigned applets" that are not allowed to access your local disk and other resources. Server removes all signatures when rebuilding applet on the server side. Also, applets are approved to run only after testing by the site team and public code review.
This is currently just a tiny initiative about 30 applets, adding several every week.