NCover

I've hooked NCover into my build script; this is a code coverage tool, which analyses the execution path of your code as your unit tests run, and alerts you to unexecuted (and hence untested) code. I'm not sure about it yet - I've yet to get anything meaningful out of the results, and am considering moving to Clover.NET - although this is altogether more powerful (and integrates with VS.NET), it looks like it may be tricky to get working with NAnt, especially as I'm using the Solution task to build with, which apparently the Clover command line tools don't yet support.

If you're using NUnit (or anything like it), then it's essential that you use a code coverage tool; otherwise, it's impossible to tell exactly how effective your tests are; just because you're using NUnit, and have written hundreds of tests, if your tests are garbage you may as well not be doing any unit testing at all.