Selenium and AspNetTest
I have previously used NUnitAsp for doing some unit testing of ASP.NET UI bits, but as it emulates a "noscript" browser, it isn't very good at testing the "scripty" bits - e.g. AJAX, drag/drop stuff. It has also not been developed very much in the last two years.
Anyway, AspNetTest describes itself as being built upon NUnit and Selenium Remote Control.
It appears that it provides a nice way to automate unit testing of the web UI stuff, and I've seen some demos that indicate that it should handle the tricky drag/drop and AJAX kinds of scenarios that are of interest to me.