Using Silverlight 3, web developers can offer multi-touch capabilities. Unfortunately, the only operating system supporting both multi-touch and Silverlight is currently Windows 7. This greatly limits the near-term importance of the feature, but if multi-touch continues to gain in popularity that will change.
Supporting multi-touch is actually quite simple. For basic operations, simply handling the Touch.FrameReported event is enough. This event includes a collection of touch points and whether the users just touched, moved, or removed their fingers. Kevin Wolf has a quick start sample on his blog.
Kevin Wolf has also a multi-touch game called Crazy Coins. In addition to Silverlight 3, this application uses the SilverSprite framework and the Farseer Physics library.

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Community comments
Platforms
by Stefan Wenig,
Re: Platforms
by Jonathan Allen,
Platforms
by Stefan Wenig,
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"the only operating system supporting both multi-touch and Silverlight is currently Windows 7"
OS X Tiger does. And what about Moblin, the next target platform of Silverlight?
Re: Platforms
by Jonathan Allen,
Your message is awaiting moderation. Thank you for participating in the discussion.
You're right, I completely missed that one. It is strange that Microsoft is putting so much emphais on Windows 7 while Apple quietly forgets that it had that technology for years. Then again , Microsoft is doing the same thing by neglecting their mobile platform's extensibility.