Examples of Form Elements

This would be a great place to list basic examples of popular form types, for example, one that has a drop-down menu, one that has hidden elements, one that has multiple groups of radio buttons, and so on.

For now, I am going to list pages that link to other pages that have examples, or that purport to have good examples, in order to create something of a clearing-house for Drupal forms examples. The problem is that these aren't differentiated on the level of complexity of the examples. Anyway, here goes something:

• There's an example in the comments of this node, of a full form with validation and submission.
• A screencast introduction to the Forms API in Drupal 6.