You will need…
This lets you write test scripts / methods etc for your code. There is a tutorial on how to use it (word doc) on their website, and if you’d like to go a step further and use NCover with it, check out my previous blog article. (NCover will monitor your testing and tell you which areas of code are being tested and which aren’t).
NUnitASP (if you’re creating web apps)
This lets you test the basic mundane, click click click around websites and test that values entered into boxes persist, or return the correct search results etc. I don’t advocate using this as an alternative to Unit Testing, but it gives you confidence that if you change your business model then your UI is still probably okay.
This is a nice plug-in for Visual Studio. It lets you run any method on the fly and debug into it. For example you have a bunch of NUnit tests, you right click into one of them and hit Run Test and it executes and shows you the results. Much quicker than running up the NUnit GUI and attaching the VS.NET debugger.