Microsoft Silverlight

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.
Silverlight was formerly code-named "WPF/E."
a exemple ?  :
The website : 

Silverlight support Windows Vista, Windows XP Service Pack 2 and Apple Mac OS X with the malority of the browsers (Microsoft Internet Explorer 6, Mozilla Firefox, Apple Safari )
Silverlight run in the browser and does not need the .NET Framework .Based on the XAML syntax you can now have a powerfull way to increase the experience on your web applications ... hummm that FLASH my mind :)

Soon, Silverlight will be able to mix the code behind (C# on .NET), so you be able to use (or not) .NET with it.