I've been playing around with NUnitAsp for about 15 minutes now, and I already like it a lot. It gives the ability to make requests to an ASP.NET web page, and then easily verify that the contents of various server controls have rendered correctly. And it plugs into NUnit, so you get that same red/yellow/green display. I even managed to make it populate a login form with my credentials, click the submit button, and verify that it redirected me to the right page. Took about three minutes to write.
In addition to being useful for doing TDD of web apps, this is definitely something that should make writing build verification tests easier. Build verification tests (BVTs) are distinctly different from unit tests, in that they are intended to work against the system as a whole, rather than against a small piece of code. So as one of its last steps, the build script can copy the newly compiled web content to the file system, create a virtual directory, then use nunit-console to run tests that hit various pages, making sure everything works okay as deployed.