I like to have all dependencies for a project in the source code repository so that the solution can build for new developers straight from a check out. After having to work sift through the NUnit binary (again) to work out what I needed rather than reference the GAC, this list is as a reminder:

nunit-console-runner.dll
nunit-console.exe
nunit-console.exe.config
nunit-gui-runner.dll
nunit.core.dll
nunit.core.extensions.dll
nunit.core.interfaces.dll
nunit.exe
nunit.exe.config
nunit.framework.dll
nunit.framework.extensions.dll
nunit.mocks.dll
nunit.uikit.dll
nunit.util.dll

This includes the UI stuff required for running the GUI - just in case there is someone out there that doesn't have resharper...