I use Google Docs and Google Reader all the time. With Google Gears, these are set to become even more useful because they'll work when I'm offline. The Reader already works this way (see this post for more), but not Docs + Spreadsheets yet. Rumour has it that this is coming. If they solve the sync problem properly, this will be seriously good: Docs is fine for note-taking and certainly good enough for blog posting. And of course they must extend Gears to GMail.
Then there's Adobe Flash and ActionScript, which has been around for a long time, is well established and has been used to create quite complex browser-hosted applications, such as Gliffy, which is (a bit) like Visio in a browser frame. Flash is cross-platform, as well as cross-browser, but still there's quite a gap between developing for Flash, and developing for the desktop. Now Adobe Labs is promoting AIR (the technology formerly known as Apollo). This looks like a warmed-over Flash, and appears to be based still on JS and HTML.
When I first heard about Silverlight, I wondered why Microsoft appeared to be tilting at the Flash/ActionScript windmill? Why would they expect developers of browser-hosted, graphical presentations to switch away from Flash? But that's not really what Silverlight is about: this is about creating a platform for Rich Internet Applications (RIAs) based on the .NET platform, which makes this potentially much more interesting, especially if development can be done in C# using Visual Studio, rather than mashing together markup and script. The runtime appears to be a a slightly reduced version of the CLR, supporting WPF/XAML as well as the traditional HTML/JS mixture. Quite remarkable, in a 1.1MB download. Rather than repeat more technical details here, I'll refer you to the Silverlight architecture summary paper.
Silverlight already runs on Mac as well as Windows, and in all the popular browsers. Could it be ported to Linux/Unix? The Mono project has proved the portability of the runtime, so it doesn't sound like such an outrageous suggestion.