Amoeba role playing game engine
   A classic role playing game engine created completely in XUL, JS, XBL, and CSS. In the classic style of early Final Fantasy games. Development still in progress.
   http://sourceforge.net/projects/amoebarpg/

Gnusto
   Interpreter for adventure games, ancient and modern, in the Z-machine format.
   http://gnusto.mozdev.org

Xulmine
   A project at mozdev.org that provides the famous Minesweeper-Game as extension for the Mozilla Browser.
   http://xulmine.mozdev.org/

Xultris
   A tetris-clone written in xul.
   http://games.mozdev.org/xultris/