Server Quest II
A while back, Microsoft release "Server Quest", a very cute, 80's-style adventure game. You can check it out at http://www.server-quest.com/. For those who're keen to try the next run in these Silverlight in-browser adventures, Server Quest II has just come out.
If you want to take it a step further and write your own, then spin over to the competition page.
On a side not, if you just feel like playing retro Commodore 64 games, Scott Hanselman has an interview with a guy who created a Silverlight 3.0 emulator.