One of the most flexible and impressive multimedia apps I’ve ever encountered is The Gong Project,from The Hong Kong University of Science and Technology.
Gong is a Java applet, and must be configured as a web application in the Apache Tomcat server. It’s actually a bit easier to configure in current versions of Tomcat than in the old one described in the Gong installation instructions. I recently set up the program on my aging Sylvania G netbook, running Vector Linux 7.0; it works reasonably well (for one user, at least) in a client-server environment.
Once the server is configured, the application may be accessed using any Java-capable web browser. Gong is nominally a threaded voice bulletin board, but it contains many other features including automated text highlighting and an API. The video below is a brief demo I put together a few years ago that shows off some of what I believe to be the most useful ESOL-specific features of the program.