Two years ago I took a quick look at XUL, Mozilla's markup language for user interfaces. It seemed like a fast and powerful way to create user interfaces. It still does, but I haven't had the opportunity to work with it. There is also an application called XULRunner that runs complete XUL-based stand-alone applications. I hadn't heard about it before today when Marc Hedlund posted about it. The Wikipedia article on XULRunner looks sketchy, so I would assume that it is not used very much, but Stephen O'Grady mentions it as an alternative to proprietary solutions for rich Internet applications (RIAs) such as Microsoft's Silverlight, Sun's JavaFX, and Adobe's Flash and AIR (formerly known as Apollo). Since XULRunner seems to be primarly a standalone desktop runtime for XUL applications, I think the comparison with Silverlight, JavaFX and Flash is a bit odd, as they are aimed towards browser-based applications. Still, XULRunner seems to be a competitor to AIR, and since there's a lot of buzz around AIR and similar technologies, perhaps XULRunner deserves more attention. I'll need to look more into it, but as I'm a slave of the hype, I'll probably look into AIR first. I hope I can get back to you with a better informed view on the subject.