SpecFlow is open source and covers all your basic needs for managing and testing specifications by example.
SpecFlow+ is a series of extensions that introduce additional features, such as a stand-alone IDE for Gherkin specifications, advanced reporting, specify and execute Gherkin in Excel, execute tests in multiple threads, and define complex filters to determine which tests to run.
SpecFlow+ is free, but requires a license. You can request the free license here.
For an overview of the differences between SpecFlow and SpecFlow+, see the comparison of features.