Metaphorical Web: Who ya gonna call? XUL!! "Interest in XUL has picked up dramatically ever since Firefox came into its own as a separate application, and the combination of ease of use, cross-platform compatibility, intrinsic web awareness and a growing community of developers makes it a technology to watch.
One problem that XUL faces is that the documentation out of the Mozilla site is sketchy at best. However, XULPlanet at http://www.xulplanet.com has several good XUL tutorials and reference documents that are up to date, if not necessarily complete or exhaustive. I would strongly recommend if you're interested in XUL that you check out that treasure trove first.
XUL is, in many ways, analogous to Microsoft's XAML, and with the recent work on the part of the SVG community making its way into the Mozilla effort, I see XUL and XAML effectively going head to head as the first of a new breed of meta-application development languages. XAML is perhaps more exhaustive, but until the Mono effort gets further along, it is not as well represented on non-Microsoft platforms. Additionally, XAML puts a lot more of its emphasis on the use of C# code-behinds, an approach that makes sense for very large enterprise developers but seems like overkill for the average programmer.
XUL may not be your cup of tea, but you owe it to yourself to at least check it out ... as an application framework, it may very well become one of the big technology stories of 2005."