The advantages of Unit Testing

• Top 12 Reasons to Write Unit Tests: A short article for people who don't know unit testing well, or are not convinced of its advantages. Written by the authors of the book Java Extreme Programming Cookbook.
• A Dozen Ways to Get the Testing Bug in the New Year: A three page article that tells how to get started in writing unit tests, and at the same time explains the advantages of unit testing. Example code is in Java and Ruby, but the article applies to testing in every programming language.
• A Revolution in the Making: When Developers Write their own Tests: A short article about why JUnit was created. It applies to any unit testing framework.
• 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.
• Learning to love unit testing (PDF): Anonther article that explains the advantages of unit tests. It especially attacks prejudices about unit testing.
• Integrating Unit Testing Into A Software Development Team’s Process (PDF): a measurement of how unit testing improves code quality.

Test Driven Development (TDD)

• Introduction to Test Driven Development: a long article that describes what Test Driven Development is and what the advantages are
• Interview on Test-Driven Development: an interview with Martin Fowler about Test-Driven Development, that's part of a larger series on Software Development.