I think Bambookit
http://www.bambookit.com is based on XUL, and goes the applet route to deliver across browsers. XUL is incredibly cool, but I think if you are looking for cross-platform/browser services, the emerging environments for XForms-based applicaions are probably safer foundations. I have been really impressed by CForms <http://cocoon.apache.org/2.1/userdocs/forms/index.html> but there are several players in this space.