Silverlight is a new Microsoft technology which is all set to rival Adobe Flash for creating rich interactive web sites. It's built on top of and is a sub-set of, existing technologies namely, .Net and WPF (Windows Presentation Foundation a new presentation sub-system which is part of .Net 3). The first version is mostly geared towards streaming media (such as video or sound although I've spotted some interesting animations, check out our own on our home page), while the next version (1.1) will allow for scripting using C# which will, I think, really open things up and allow Silverlight to rival Flash in what it is capable of.