The regression-test bonus
Defining and automating your acceptance criteria up front makes a lot of sense. Not only does it provide clear goals for developers, it also gives excellent visibility into what feature are being implemented, how they will work, and how the project as a whole is progressing. And, as a bonus, ATDD also provides a broad set of regression tests.
Many open source tools exist to help you implement an ATDD strategy in your project -- I've put together a compendium of resources. Although you can use conventional unit-testing tools for ATDD, dedicated ATDD tools provide a stronger emphasis on communication and reporting, which are key parts of the ATDD approach. And for Web applications, automated testing tools based on the page-objects pattern are an excellent choice when it comes to implementing the tests themselves.
Read more about Tools & Methods in JavaWorld's Tools & Methods section.