Java is a programming language with long history, and due introduction of Java applets long time ago used to be the important platform for providing interactive educational material. While other uses (site decoration, enhancing user experience, etc) are currently in strong decline, educational Java applets remain because often it is complex to convert them to some other platform; also because the alternative platforms may not fit wanted all requirements. Many good applets are written by university professors that have no time nor desire to learn new languages and rewrite they material from 1990ies into something more fashionable.
1 Java applets as educational media
The idea that Java applets could be an excellent media has been proposed as early as in 1999, supporters being convinced that:
- the unsupervised studies of static content may not be sufficient for understanding complex concepts, and
- the distance learning does not scale well enough if the human teacher must interact with every student.
Java applets seem to be the most useful to illustrate mathematical, technical, electric and some other subjects, from interactive function graphs till mathematical models with advanced visualization. Unlike animations, applets can interact with the user, allowing active experiments. They are fast enough for non trivial visualizations that are computation intensive. They are more portable, secure and website-integrable than standalone applications. Long time ago, the old applet technology has been upgraded to Java Web start that is less dependent on sometimes problematic decisions taken by the web browser development team. Also applets can be easily launched through web start.
Actually, in somewhat 1998's the good part of Java - related talks were talks about applets, while mostly focusing on visual effects. At one time some web pages contained dedicated applet per single navigation button! This direction seems exhausted now: applets are overkill just for "eye candy" that can be easier produced with alternative technologies.
However a completely different group of applets started to emerge during the years. Lots of applets were created for educational and similar purposes. It is even possible to suspect that this is one of the most successful areas of they use. Apart separate applets, there are also some big projects, for instance, the complete suite to assist the course on differential equations, a portal devoted to heart physiology, a several suites of stunning applets on physics and lots more.
2 Acceptance of Java by free content movements
From the other side, despite of the availability of the source code and growing number of FOSS projects, for a long time Java has been considered 'non-free' and hence poorly suitable for enthusiasts of Free content. The problem was well understood at that time, with alternative GNU Classpath being one of the top priority projects in FSF. The interest raised after Sun released OpenJDK under Open Source license. The idea to include applets as part of Wikipedia-like encyclopedia has been published in JavaWorld in 2008, and the Wikversity project (LabsWiki) has been launched near at the same time. It aims to create both Java applet support for MediaWiki (wiki system that Wikipedia uses) and applets themselves. The project page at  contains the code of some applets that were all later included into Ultrastudio.org as well.
While never focused on this mission, SourceForge can also be viewed as a first prototype of applet encyclopedia, as it hosts source code, executables and startup pages for many various applets. Good half of the applets in Ultrastudio.org come from SourceForge as they licenses are always appropriate.
3 Wikipedia effort
The idea was remembered during the 2010 year Wikipedia strategic planing  where it caused a hot discussion without obvious consensus . The very similar article has also article has been presented in Advogato , understanding, that Wikipedians and FOSS are not necessarily the same community. While these discussions did not bring to Java introduction in Wikipedia or anything the like, they anyway brought many useful feedback.
Ultrastudio.org  is a logical continuation of the educational Java history. This project aims to collect, review and fix the freely available educational Java content. At the time of writing, over 100 such visualizations are available on the project site, many for them fixed and enhanced by the team. Most of them are truly open source (as a rule, GPL license) but some professors insisted on bilateral agreements, publishing source and ready to run apps as "allowed to use in Ultrastudio.org project without restrictions". While applets are available only on the project site, the published web start links can be easily added to any educational project (following the link launches the application), making the project a useful resource when building the educational website.
- Pankaj Kamthan (1999) Java Applets in Education
- The d'Arbeloff Interactive Math Project
- The virtual hearth
- Paul Falstad applet suite
- Brainflux applet collection
- Physical applet overview maintained by prof. Michael Fowler
- R. Stallman (2004). Free but Shackled - The Java Trap.
- Randall Scarberry (2008). Add Java extensions to your Wiki. JavaWorld
- Wikiversity project to put applets to encyclopedia
- LabsWiki project at SourceForge.net
- Java Applet Support strategic proposal
- Discussion on Wikipedia proposal
- Java applets for Wikipedia - what do you think?. Advogato article
- Ultrastudio.org - project website.
- Web start link that should start a Hydrogen Orbitals visualization on the user desktop. This application is under GPL.