Mandel
This applet displays the Mandelbrot fractal set. Clicking on the image
will zoom in. The "Options" bar allows you to change the maximum number
of iterations, reset the image, or get help information. This applet
illustrates how an image can be periodically updated while it is being
computed.
Performance hints: the lower the maximum number of iterations,
the faster the computation. Also, the black region (the Mandelbrot set
itself) is the
slowest to compute, so if you zoom includes a lot of it, computation will
be slow.
The source is here.
If you're looking for more information on fractals, I have reviews
of some fractal books.
WebCounter: 
Ken Shirriff:
shirriff@eng.sun.com
This page:
http://www.righto.com/java/mandel.html
Copyright 2000 Ken Shirriff. Last updated 3/13/2001.
Java and other Java-based names are trademarks of Sun Microsystems,
Inc. and refer to Sun's family of Java-branded technologies.
Use of this software is subject to the terms of the
license agreement.