Overview 

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.

Downloads

Here’s the runtime itself:

• Silverlight 1.0 RC1 / Windows (direct link)
• Silverlight 1.0 RC1 / Mac (direct link)
• Silverlight 1.1 Alpha Refresh / Windows (direct link)
• Silverlight 1.1 Alpha Refresh / Mac (direct link)

Some tools for building Silverlight applications:

• Visual Studio 2008 Beta 2
• Visual Studio Extensions for Silverlight
• Silverlight 1.0 SDK / Silverlight 1.1 SDK
• ASP.NET Futures CTP
• Expression Blend August Preview
• Expression Media Encoder Template Updater

Resources 

You can learn more about Silverlight from the Silverlight community site.

Finally, there are a couple of blogs that you should watch if you’re getting into Silverlight development – Barak Cohen (product marketing manager for Silverlight), Mike Harsh and Joe Stegman (from the Silverlight development team).