Depend on what you would like to achieve.
You can use web test built in the Visual Studio Tester Edition. It's quite good and easy to automate. You can use external data as a test data source and it integrates nicely with VS.
There is also test tool by Automated QA (forgot the name) which looks good but expensive.
And there is Selenium. That's the one we are using in Symantec. The biggest advantage is that it actually uses a browser you want to test. VS mimic a browser by changing http request parameters only so you may not be able to test your site for cross-browser compatibility. Selenium on the other hand uses browser and automates it so you can actually test your site in IE, Firefox etc. It can be also integrated with VS unit tests so you can see test results in VS.
So I would recommend Selenium or VS.