Is there a free Visual Studio addin for Nunit?

You can create a blank project (Choose console application for example) and in the property of the project you can select DEBUG tag and select "Start External Program". Put the path of Nunit. Then, in the start option, the command line arguments select the DLL that contains all your tests (mine is always in the nunit\bin...). Then select "enable unmanaged code debugging" and you will be able to start the project inside VS and even use the debugger step-by-step.

This is a free solution.

Now you can use Gallio: it's open source. www.gallio.org