building a completely data-driven test harness that lets you express common types of test cases entirely as data files in a directory, without adding any additional code. Done right, this lets you add new test cases directly in a familiar format, and even directly translate bug reports or interactive tests into test cases.
(A coworker calls this pattern a "zoo", because you get one directory containing all of your exotic animals, and I've adopted that nickname)
Related to testing-concept-miniformats-for-test-input-and-output