NCover is a pretty good code coverage tool for .NET. You can use it to determine which parts of your code aren't tested by unit tests, thus letting you know if you need to write more tests.
A common problem identified by code coverage tools is that you have blocks of code that you wouldn't encounter under regular circumstances, such as Exception blocks that are intended to catch unforseen errors or error conditions that occur rarely. Due to insufficient testing for those blocks, you could be releasing some untested code that could result in a bug.
NCover can be run with pretty much any kind of tests - you just need to provide them as parameters to the command line NCover tool or browse to find the executable for the graphical tool so you aren't limited to any single testing framework, such as NUnit.