Silverlight by Microsoft, helps you create rich web applications that run on Mac OS, Windows, and Linux.
Silverlight 3 introduces major media enhancements, out of browser support allowing Web applications to work on the desktop; significant graphics improvements including perspective 3D graphics support, GPU acceleration and H.264 video support; and many features to improve RIA development productivity.
To fully integrate all the .NET developer tools, Visual Studio 2008, Visual Studio 2010 and Visual Web Developer Express support a fully editable and interactive designer for Silverlight.
New features in Silverlight 3 include:
- Support for Higher Quality Video & Audio
- Empowering Richer Experiences
- Improving Rich Internet Application Productivity
- Enhanced Data Support
- Improved performance
- Out of Browser Capabilities
See detailed list here