Silverlight is a cross-browser, cross-platform plug-in for delivering the next
generation of media experiences and rich interactive applications (RIAs) for
the Web.

This could be the Flash killer, though it will probably take a few years to get there. I haven't actually seen the development environment for this, but if you can use .Net (or something very similar), Visual Studio 2005, and Expression Suite to build it, then this will be very compelling for a large number of developers.