I'm not sure how efficient a use of open source programming resources it is, but the "Sharp Widget Toolkit #WT (SharpWT)" certainly sounds interesting.

The Sharp Widget Toolkit (SharpWT or #WT for short) is a port of the Java SWT (Standard Widget Toolkit) to C# and .NET. The initial porting effort started when the SharpDevelop team wanted to move away from Windows Forms to a different UI toolkit that would allow for portability but still have the native look and feel as well the performance on the platforms it runs on. Thus after some evalution we settled for porting SWT to C# - because SWT is already proven to be a great cross-platform UI toolkit as shown by IBM's Eclipse project."