While this appears to mainly be a proof of concept, and bit rough around the edges, I still think it's insanely cool. You can open it in FireFox or any of the Mozilla browsers. In Mac OS X it looks and feels like an OS X App: all the controls and UI elements are native to the OS. Here's a screenshot.
This application appears to leverage the Amazon Service API.
XUL Apps can be loaded over HTTP/The Web, or opened locally from a user's desktop. In each case, the application looks, feels, is responsive and behaves just like any other regular desktop application.
XUL is an open, community-driven standard and framework, that builds upon mature Web Standards.
XUL Applications will run in any Mozilla browser.
All Mozilla browsers, including FireFox, run on all computers, all platforms.
Yet another compelling reason to give FireFox a try :)