NunitForms

NUnitForms is an NUnit extension for unit and acceptance testing of Windows Forms applications.

our NUnit tests can  open a window and interact with the controls. Your tests will automatically manipulate and verify the properties of the gui. NUnitForms takes care of cleaning up your forms between tests, detecting and handling modal dialog boxes, and verifying that your expectations for the test are fulfilled.

Go to http://nunitforms.sourceforge.net/

See also these related projects.