UPDATE: Given WatiN has been stagnant for over a year now, I would direct anyone that needs web ui tests towards selenium, it is in continuous use & development by many contributors, and is actively used by Google.
WatiN is the best that I've found. It integrates into Visual Studio unit testing or nunit & you can do pretty much anything you need in the browser (click links, submit forms, look for text/images, etc.)
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