I am familiar with nunit for unit testing of the business layer however I am looking now to automate the test of the win forms gui layer.
I have seen watin and the
AutomatedQA's TestComplete is a good testing application to automate GUI testing. It supports more than just Windows Forms also, so you can reuse it for other applications. It is not open source and this is the best that I have found. I haven't seen an open source equivalent to WatiN. It does have a free trial, for you decide if you like it or not. The main reason I went with it, is that it really is cost effective, compared to other testing applications.