How do I get all available form field types? How do I get all form field types (e.g. "textbox," "select") available in Drupal 6?
The core fields are provided by hook_elements so you can just invoke that hook to get a list of all field types in the site using module_invoke_all():

 $all_types = module_invoke_all('elements');


The array keys of $all_types are the machine names of the elements, each of those points to another array containing the element information.