Fit is a framework that helps the business get involved in specifying the requirements that end up being the testing. When the business can use Word and Excel to generate their own test cases, connected to a simple fixture and can be run by developers, customers and managers on demand, the potential for something great arises.
Encouraging the business to get involved at an early stage means that the business refine their requirements further.
Fit Tests can tell us useful things. Where a FIT test fails, its an indicator that we need more code to cover business requirements.
Fit complements xUnit, the fixtures are similar but the test data, state and structure is all defined in the business supplied doco’s. The fixtures are simply glue from these documents to the tests.
Automating Business Value with FIT and Fitnesse from InfoQ
Fit: Framework for Integrated Test
And when regular XP wont do, get Industrial XP. Nothing to do with FIT, just a site mentioned in the first link.