Some of you may have heard about it. It's Microsoft's response to Adobe Flash and from what I've seen it's pretty sweet. You can get the things here (Totally read in less than 30 seconds!) and showcase it here. I'm amazed at how fluid it runs unlike Flash which lags quite a bit.
I've had an eye on Silverlight for a while. I believe that I can develop for it in C#. Makes me wonder if it uses GDI+ compatible drawing techniques...?
The Register have posted an article on Silverlight Pros and Cons, which makes for a quick but interesting reading. As a C# developer, it seems to be a more natural choice for me than Adobe Flash/AIR, despite it's lack of maturity. The upcoming Silverlight 2 appears to be an answer to a number of shortcomings of the original release, though.