I will put a few links related to ASP.NET unit testing.
Initially I saw reference to WATIR from Scott Hanselman’s post Integrating Ruby and Watir with NUnit .
Recently I’ve found WatiN Web Application Testing In .Net, including articles WatiN – Web Application Testing In .Net – The Code Project – ASP.NET and WatiN Test Recorder – The Code Project .(The home page for WatiN Test Recorder is here)
There is interesting article by Jeremy D. Miller with comment from ScottGu recommending WatiN .
Using regular expressions in WatiN (e.g. for asp.net long IDs)
Also WatiNFixture is a custom fixture for FitNesse – acceptance testing framework.
VSTS ASP.NET tools
From MSDN Overview of ASP.NET Unit Tests it seems, that you can only test business classes in App_code. However there is a blog post describing Testing the page object for an ASP.NET site .
To simulate ASP.NET server functionality(i,e.HttpRuntime I am using class based on Haacked’s HttpSimulator. May be http://www.codeplex.com/plasma should be considered.