Microsoft released a web based version of The World Wide Telescope today. The World Wide Telescope is a Microsoft Research projects that basically takes all of the high quality images of the known galaxy/universe/etc… and stitches them together into an amazing user interface. The Desktop client is very impressive and is a great experience if you have a powerful video card and processor.

One of the great things about this is tat users can create tours of tours of constellations and other celestial bodies and share them with the world.
It create more of a community feel and gives you a reason to come back often.

The web client can be found here:
http://www.worldwidetelescope.org/webclient/ .

It looks like it supports everything found in the desktop application. The web client is running off of Silverlight. Silverlight is an animation / rich interface platform that competes with Adobe Flash. To be honest this is the most notable use of Silverlight that I am aware of. It is pretty impressive none the less.