JFLAP is a tool developed by Susan Rodger and others, for the visualization of the machines and languages studied in a typical Theory of Computation course.
For more about JFLAP, see http://www.jflap.org.
For a tutorial on its use, see http://www.jflap.org/tutorial .
Download: You must have Java 6 (a.k.a.
Java 1.6) or later installed.
Download this JFLAP.jar file and double-click it.
(This worked for me in FireFox 3.5, but not in IE 8)