Test Driven Development

Forces strict test-driven development by requiring tests before any implementation code. Won't let you write production code without a failing test first, and actually makes you delete existing code if you wrote it before the test. Walks through the red-green-refactor cycle with mandatory verification steps at each phase. Catches all the common rationalizations like "I'll test after" or "keep it as reference" and shuts them down. Good for breaking the habit of testing after implementation, since it's surprisingly hard to stick to true TDD without external enforcement of the discipline.