Microsoft Silverlight may be seen as a lesser rival to Adobe's AIR as a platform for developing RIAs (rich Internet applications), but with the new Silverlight 3, that may be about to change. Silverlight 3 was announced alongside the Expression Studio 3 suite of applications for developing in the format at Microsoft's MIX conference in March.
Microsoft Silverlight 3 is catching up to the capabilities of Adobe Flash, Flex, and AIR in all the areas where Silverlight was behind. Silverlight 3 applications can run in or out of the browser, online or offline, with much improved audio, video, and 3D graphics.
Recently I've been hearing from Adobe on a regular basis about adoptions of the Adobe Flash Platform by large media organizations, such as the BBC's iPlayer and Channel 4's 4oD services, for streaming media content to the Web both live and on demand. I've been hearing rather less from Microsoft about Silverlight adoptions, outside of a very few big brand projects such as online versions of Playboy (below) and Rolling Stone, and criticism of companies who ditch the platform for Flash and AIR.
I think that part of the reason is that Adobe leapfrogged Microsoft last winter in the area of media support, particularly H.264/Advanced Audio Coding (AAC) audio and full HD video playback. These and many other capabilities are included in Silverlight 3, which is currently in a beta that does not include a go live license, but will be released in July.
Another area where Flash and Flex were ahead of Silverlight is Windows and Macintosh desktop operation. A number of desktop Flex/AIR applications have become popular, especially Twitter clients; examples include TweetDeck, Twhirl, DestroyTwitter, and Seesmic Desktop. (Let's ignore the memory leak issues they all have in common for the moment.)
Out of the browser
Silverlight 2 didn't have a viable way to run on a desktop; the best a developer could do along those lines was to build a desktop WPF (Windows Presentation Foundation) application based loosely on a corresponding Silverlight RIA (rich Internet application). Silverlight 3 addresses those issues very nicely, with easy ways to install Silverlight applications on a desktop, update them in place, detect Internet connectivity state changes, and store information locally and securely.