A Microsoft project supporting animation, graphics, interaction, and sound. Offers some relatively advanced features, and has some nice properties - for example, text in MicrosoftSilverlight can be indexed. 

One feature of MicrosoftSilverlight: it offers alternatives to JavaScript for manipulating DocumentObjectModel. Both DLR and CLR languages are supported as of Silverlight 2.0. IronRuby, IronPython, JScript (Microsoft's JavaScript), and CeeSharp are named explicitly.

A related project, Mono's Moonlight, re-implements MicrosoftSilverlight with some token support from Microsoft, but faces distribution challenges: anyone may download it from Novell, but only Novell may package Moonlight with a Linux distribution. 

See Also
* http://silverlight.net/ 
* http://www.go-mono.com/moonlight/
* http://en.wikipedia.org/wiki/Microsoft_Silverlight
* LimitsOfHtmlStack