NUnitAsp is a tool for automatically testing ASP.NET web pages. It's an extension to NUnit, a tool for test-driven development in .NET.
Once you have an automated suite of tests, you'll never go back. It gives you incredible confidence in your code. That confidence allows you to code much faster, because you can make risky changes secure in the knowledge that your tests will catch any mistakes.
NUnitAsp is for unit testing ASP.NET code-behind only. It's meant for programmers, not QA teams, and it's not very good for QA-style acceptance tests. It only tests server-side logic. JavaScript and other client-side code is ignored. But if you're using ASP.NET, it's an essential part of your programmers' toolset.