People write applets to do all kinds of things, usually focused on a graphical and/or dynamic presentation of data from the server. There's one particularly good applet which you can easily find on the web. The applet is at http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/index.html
Known as the "Powers of 10" demo, it was written by Matthew J. Parry-Hill, Christopher A. Burdett, and Michael Davidson of the National High Magnetic Field Laboratory at Florida State University. The applet is based on the book "Cosmic View: The Universe in 40 Jumps" written by brilliant Dutch engineer and educator, Kees Boeke.
The "Powers of 10" applet presents a series of images, starting with the galaxy at 10 million light years from earth. Each image brings you, the observer, 10 times closer to earth.
After about 15 such zoom-ins, there is a recognizable planet earth in the picture. Further zooms bring you to the Western hemisphere, the state of Florida, to a tree by the National High Magnetic Field Laboratory. You then zoom into a leaf, a leaf cell, DNA, and sub-atomic protoplasmic globules. "Powers of 10" is an interactive Java-based tutorial on comparative sizes, fun as well as educational.
\ Michael used a couple of interesting techniques to speed up performance. Only a couple of the images are downloaded before the applet starts. The rest are brought from the server as the applet is running, so the user does not have to wait. The "zoom in" effect on an image is done by re-drawing the images at different sizes rather than inefficiently using a succession of different-sized images.