Q:

Can NUNIT, a unit-testing framework, be used for integration testing? 

Can NUNIT, a unit-testing framework, be used for integration testing?  It seems to be working that way for me now.  Are there other automated testing frameworks out for integration testing? 

A:

Yes, you can use NUnit for integration testing.  For that, you may take an advantage of attributes like [Setup] and [TearDown]. 
Other .NET unit test frameworks which can be used for integration testing as well are:

MSTest
xUnit. NET

For more testing framework types (UI testing, database testing, web testing, acceptance testing, etc.),  you may check List of Automated Testing (TDD/BDD/ATDD/SBE) Tools and Frameworks for .NET.