White is an open source .Net library designed to support Unit Testing of Windows GUIs. I used it for the first time this week. The results were good enough to recommend using it again. The performance was a little slow, so I wouldn't want to have lots of tests. I prefer to avoid UI Unit Tests by separating the presentation from the logic. However, sometimes you need to unit test an interaction. For those cases, White can be useful.
I'm not going to blog an example here. The documentation for White has some examples. See Getting Started.