It is desirable to know test code coverage sometimes. It indicates quality and amount of tests and helps to ensure that TDD going right. This even could be one of the metrics for release acceptance (code coverage should be no less than 75%, for example).
NCover is a free tool for test code coverage metric generation (for .NET projects). It integrated pretty good with NUnit and within 10 minutes or so you may confugure it and get useful report.
Install NCover and NUnit and add ncover.console.exe and nunit-console.exe into PATH environment variable. Open comand prompt and to to the folder where your DLL with tests located. And just run the following command
> ncover.console /c nunit-console Tests.dll /a Project1;Project2;Project3
By the way, default stylesheet is very bad. You can get more better one here, Replace original Coverage.xslt with new in NCover folder, it will be copied each time when ncover runs.
If you use NAnt, you may add NCover report generation as a task.