While Adobe has been getting most of the press recently for their Flash 10.1 RC, Microsoft has quietly announced their plans to release the final version of Silverlight 4.0 as early as next week. This major update will provide more fundamental changes than prior iterations, including Google Chrome support, better performance (up to 200% over Silverlight 3), improved security with digital signing and sandboxing, and greater control for developers.
Silverlight will also be the primary development environment for Microsoft's Windows Phone 7 Series platform -- the full programming model, not a mobile version -- so those interested in writing apps for these devices may want to tune in to see what's coming. The company is offering a Windows Phone 7 development toolkit for free, including Silverlight, Expression Blend for Windows Phone and a preview of Microsoft Visual Studio 2010 Express.
The release candidate of Silverlight 4 has been available for download since mid-March 2010, with Microsoft releasing the preview code at the MIX10 conference in Las Vegas. A quick overview of new features can be found here, or you can opt for the gritty details in this technical white paper (Word 2007 format).