Lately I've been taking a cue from Extreme Programming, and am writing tests cases for the functionality I'm about to implement. (The goal is to write test cases before code, but sometimes this isn't practical, and I have to grow the test cases along with the code.) I have a simple test harness that I'll write up later, and a number of test scripts or inputs, and "blesses" test outputs. (The harness generates outputs, and compares them with their "blessed" counterparts.) These all have to live somewhere, so I keep an additional directory in the hierarchy for them:
/appl/project/tests/