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.