Silverlight. Project Tuva: Enhanced Video Player Home. Getting Started with Silverlight. Despite all the wonderful things you can say about HTML, CSS, and JavaScript, I think most people doing a lot of web-based development would agree that they form a pretty poor environment for developing modern sites and applications.
If you care about your content working on most web browsers (or even just Internet Explorer and Firefox), accommodating their differences can be maddening. Many techniques and JavaScript libraries have been developed and shared over the years that can reduce this frustration, but none of them are silver bullets. In addition to browser differences, the graphical capabilities of HTML are too limiting for many user experiences that people want to create.
Drawing a simple line, incorporating video, and a number of other things are extremely difficult or impossible with HTML alone. Considering these issues, it's no wonder that Adobe Flash has been so successful.