Silverlight is a cross-browser, cross-platform plug-in for delivering the next generation of media experiences and rich interactive applications (RIAs) for the Web.

The Silverlight 1.0 Beta has a go-live license that implies it can be used for commercial purposes.

Microsoft Silverlight 1.0 Beta

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Silverlight 1.1 Alpha is intended for developers and designers interested in learning more about how to build experiences with Silverlight using both managed code and Microsoft JScript.

Microsoft Silverlight 1.1 Alpha

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Microsoft Silverlight homepage