The Mozilla XUL Periodic Table is great stuff. If you want to see what XUL elements are available to you, how to use them, and what they look like with different attributes, this is the place to go (via Chu Yeow Cheah).
Mozilla’s XUL is really simple stuff. When you tie it in with Mozilla’s built-in support for XML-RPC and SOAP (check the articles on Using the Mozilla SOAP API, and Mozilla SOAP/Google API Demos), you’re well on the way to creating decent cross-platform applications. The biggest problem of course, is that we’re still waiting for the XRE (although I guess the GRE is usable).