Just a quick pointer to an article by Richard Allen. He is showing how easy it is to extend WatiN with extension methods and gives you some extensions he found valuable to use. Have a look at http://richallen.blogspot.com/2008/08/watin-ajax-and-some-extension-methods.html.
I've done some nice stuff with extension methods as well to add support for your own controls (or third party controls) "to" the WatiN API. My blog post about it is still in draft version but will be posted soon.
Another great blog post by Richard is about writing a framework to wrap WatiN and make your (functional) test code more fluent and domain specific. He calls it the WatiN Controller Framework but is looking for an other name (what about WaCoFra, kidding). For does who are interested, follow this link http://richallen.blogspot.com/2008/08/watin-controller-framework-part-i.html