Bob Powell. Lots of stuff on GDI tips and tricks.
Code Project. A great source for open source projects. But buyer beware--not everything here is vetted by the editors, so occasionally you'll run into some mediocre articles.
DoFactory. This is probably the single most useful website on design patterns and it's all in C#.
IDesign. Juval Lowy, who wrote Programming .Net Components, owns IDesign.
Microsoft Patterns & Practices. Some useful articles can be found here.
WindowsClient.NET. This is quite a useful site if your focus is Windows Forms.