Need to write a module that implements a form that needs some AHAH magic? Then use the awesome AHAH Helper module.
Here's a quick example for those of you who don't know what I'm talking about: let's say you have a form you've defined in your module (using FormAPI, of course) where you're asking the user to list their favorite types of trees. The form can initially supply 3 text fields for 3 types of trees, but what happens when the user wants to enter more than 3 trees? Using AHAH, you can provide a button on the form to "Add more tree types" - when the user clicks this, a new text fields is added.
Drupal 6 core has a built-in mechanism for doing this, but the AHAH Helper module greatly cuts down on the amount of code you have to write to get it done. The result is a very clean and easy-to-understand implementation.