Viral video sites are everywhere: it seems that everyone has an embarrassing moment or unusual performance that they want to share with the world at large in the hope of becoming the next internet celebrity. All those videos eat up a huge amount of bandwidth, of course, and content sites are almost universally looking to advertising as a way to both pay for the hosting costs and make a decent profit. Some sites force you to sit through a bumper ad before you can watch the content you're looking for; other sites show regular text or Flash ads around the edges of the screen. What would be ideal is something that is contextual to the video, not so intrusive that it frustrates the user (like the bumper ad), but also not so far out of the user's focus that they don't even notice it's there.
In that vein, I thought I'd put together a quick screencast that shows how you can use Expression Blend and Silverlight to do static image ad overlays on videos, with animation to toggle the advert on and off. This kind of thing is straightforward to accomplish with Silverlight and it meets the criteria above. Of course, this technique isn't just relevant for advertising - you could have the animations triggered by markers within the video, or used to display live updates in a ticker along the bottom of the screen.