Regarding MVC, this is more of a tutorial, but NerdDinner is a pretty good app / tutorial. MVC is brand new, so good well-designed, free sites are not so common. I would like to see a few. There's a pdf with NerdDinner that goes through the app.
For web forms, XD World Recipes is a well-designed n-tier application. This is a bit dated (2 years), which makes it enjoyable to learn the basics of N-Tier before we got all Linq syntactic sugar, etc.
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