Adobe's Apollo is due early next year, as is a more functional (read: useful) XULRunner from the Mozilla foundation. But first out of the gates is Microsoft with their cross platform web content runtime framework: you can now try out WPF/E. Read more here.
At least from a "promise" perspective, WPF/E more squarely what Avalon - now WPF in Vista - or even ChromeEffects (for you old-timers) was going to offer, but, significantly, this is cross-platform from the get-go. The "E" is for "Everywhere". And just to prove it, the CTP is available for Windows and Mac OS X (well, Safari on OS X anyway).
I'll post some thoughts later this week after playing with it. As I've mentioned before, the evolution is towards general purpose computing technologies to make rich application distribution instantly ubiquitous. There are some significant and interesting technology choices that imply certain classes of applications and uses... more on this in the future. Confusingly, WPF will ALSO run in your browser and enable web applications but only on Windows, and its, uh, somehow different than WPF/E, and, uh, richer (?) somehow... though neither is a subset of the other... yeeeeahhh.... quick over there! Linux is eating your IP!!!! .....
Of course, still an open question is what "rich" really means and if people really care at all ... :)
My (biased) guess is that it does, but our current generation of applications and infrastructure (not to mention imagination) just isn't there yet. After the fact, I think we'll go "of course!" (iPod anyone?)
(and oh, btw, this was the idea with Boxely and the OCP, too, but... cie la vie :))