jsforth -- Forth interpreter in JavaScript -- best viewed with Firefox, Mozilla, Netscape, Epiphany, Galeon.
Avoid Internet Explorer, it really sucks with this program. Recently some browsers changed to not passing backspace chars, entered into a form, to the keyhandler dealing with input. This results in jsforth not able to receive Backspace keys, entered into the form, while form contents are still updated (letting chars disappear). I have no solution for this at this point.