I wrote a tutorial on XUL for IBM. It's a very introductory look at XUL. You can do some seriously crazy programming in XUL. The focus of the tutorial is on how web skills can be leveraged in XUL. Adobe AIR is getting a lot of press right now (and deservedly so) for letting web developers bust out some desktop apps. XUL lets you do the same thing. It's definitely a little more complicated than AIR, but also much more powerful. XUL gives you access to a lot more desktop resources than AIR does. Plus, if there's something that it doesn't already give you, then you can just write some native code and expose it via XPCOM. I think a lot of AIR developers are already begging for this kind of extensibility.