Silverlight

Microsoft Silverlight
Microsoft® Silverlight™ is a cross-browser, cross-platform plug-in for delivering the next generation of .NET based media experiences and rich interactive applications for the Web. Silverlight offers a flexible programming model that supports AJAX, VB, C#, Python, and Ruby, and integrates with existing Web applications. Silverlight supports fast, cost-effective delivery of high-quality video to all major browsers running on the Mac OS or Windows.

Silverlight will compete with Adobe Flash and the presentation components of Ajax. It also competes with Sun Microsystems' JavaFX, which was launched a few days after Silverlight.

Version 1.1 will include a complete version of the .NET Common Language Runtime, named CoreCLR, so that Silverlight applications can be written in any .NET language.

Links and info:
Get Silverlight
Wikipedia Entry
Silverlight Homepage