Paint.NET 2.0
And while we’re on the topic of real-world .NET Smart Client applications, you can’t not take a look at the Washington State University course work resulting in Paint.NET, an all-.NET implementation of a painting program that was initially aimed at merely replacing MS Paint Brush, but has long ago far surpassed it with painting features found only in commercial painting applications.
And, like SharpDevelop, if you like what you see and want to know how a feature was implemented, you have only to download the Paint.NET source code.
Also, don’t forget to check back in mid to late January for Paint.NET 2.1!