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.

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

Gnusto
Interpreter for adventure games, ancient and modern, in the Z-machine format.

Xultris
A tetris-clone written in xul.