The advantages of Unit Testing

• Test Infected: Programmers Love Writing Tests: This article starts by explaining the problem of programmers who don't want to write tests, and then shows how easy it is to do with a unit testing framework (JUnit, but the article applies to any programming language). Written by Kent Beck and Erich Gamma, who wrote several books and articles on unit tests and Test Driven Development.
• Effective Unit Testing: article for people who have strong doubts about unit testing.
• Unit Tests - lessons learned: A short article about what unit tests are, with advice on how to do it well.

Test Driven Development (TDD)

• Introduction to Test Driven Development: a long article that describes what Test Driven Development is and what the advantages are