Automated testing
Automated testing is increasingly the direction Drupal development is taking, due to Dries' recommendations for the Development Cycle of Drupal 7. To summarize, by providing complete test coverage for Drupal 7, we can extend our development time and decrease the code freeze time- this is the power of testing. Testing means bugs can be caught automatically and expediently, which makes it easier for code reviewers. Also, more time means we can concentrate on implementing features. In short, SimpleTest is a great thing for the Drupal project, especially in the long run.
We have decided to use the SimpleTest PHP Library for our test coverage (for now- this is subject to change, as Dries has not yet made an official decision).
So, if you're interested in helping, where's the best place to start? Jimmy Berry, or boombatower, one of the SimpleTest module maintainers, has created a handy list that shows the progress that has been made towards 100% Drupal core test coverage for Drupal 7. Since there are currently tests that haven't been written, sign up for a test, and use the resources below and in this handbook to get started.
A word of advice: To write a test, you have to think like a tester. You have to exercise all the important branches of your code and poke on corner cases. At first, this can feel foreign but please persevere - the rewards are great for both yourself and the Drupal community.
The following resources provide some additional information on unit testing and Drupal development: