The first thing I’ll mention is YUI tests. If you’re a TDD developer (and if not, why not?), YUI unit tests are the way TDD should be. The change/test cycle is very fast (to run the tests, hit “reload” in your browser). And since they’re proper unit tests, they feel like a safety net when you start to make changes.
They are a good way to test your model code and HTML changes. Other members of the Launchpad team have used them to do more integration-style testing, as well. It’s somewhat difficult to set up I/O dependent tests, but that will encourage you to make most of your code I/O-independent.
For information on how to get started with them, see: Developing with YUI.Test