Look out Flash, here's come another ray of light, called Silverlight. Silverlight is Microsoft's answer to rich web applications which previously dominated by Macromedia Flash. I tried some of the samples, looks like it can do pretty much what Flash can, like, play games, watch movies and enhance users's interaction on your website.
Apparently, Silverlight would work across platforms and browsers. Developers on the other hand, would need the .NET framework to develop but there's a piece of software that end users need to download to view such Silverlight enabled websites.
If you would like to download and test this, check out the CTP. The software is still under development and is not final yet though.