JFLAP, an excellent (free)
graphical automata and Turing Machine simulator in Java, downloadable for
your own PC.
GAP/Automata, an excellent
(free) CLI automata package, available on dwarf (command gap). To
use the automata package, run gap, then give the command
LoadPackage("Automata"); The documentation is available at http://www.gap-system.org/Packages/automata.html.
Turing's
World, an excellent (commercial) Turing Machine simulator for Mac OS
Classic.