Update 22 July '07 : Came across Silverlight Tetris game and its Source code.
(Thanks to frankarr)
Some of the Silverlight stuff posted above may not work with Silverlight RC, if you have installed the RC unless those apps have been updated for RC.
Update 1 Aug '07 : Richard Z's Jelly sample graphs in various styles, bar, pie chart etc. based on Silverlight. This has been updated to work with Silverlight RC also. Check out these cool graphs samples. Source code here .
Also you can check out this video which shows, talk with Creators of First Non-MS Silverlight game called Zero Gravity.
Zero Gravity: Silverlight based puzzle game , where they talk about the game, its various levels and also talk of releasing the source code at some point of time.
Hope you have great time trying out these amazing applications and games which I must warn you, some are addictive.
Disclaimer : As there are many links provided from different sources, at some point of time, some links may be broken.