Does your code work?  How do you know?  Can you prove it?

If you don’t have tests for your code, it sucks. And I mean comprehensive, fine grained, programmer tests (aka something like unit tests), as well as higher level functional and integration tests. Tests that are automated. Tests that are run routinely. With the programmer tests run after any change to the code.

Without adequate tests (in terms of both quantity and quality) you simply can’t have confidence that your codes works as required. And even if you are confident with your work, why should anyone else be? What about people who work on the code later? How do they know it still works after they make their changes?

Of all the tools and methods out there, which are your favorite?  What is the least useful?