Vivek Singh's automated functional testing tool White has made it to #6 in the list of top projects (by page view count) hosted at CodePlex for 2008.
White allows the automation of Windows applications and so ends up being an excellent choice for the functional testing of thick clients. If you're building a Win32, WinForms, WPF or SWT based application and need automated testing, you should give White a shot.
Vivek is also on the lookout for folks interested in contributing to the project. If you're once of them, you should consider submitting a patch or three to get started.