Hierarchical Select implementation allowing to choose content types and their fields in a user-friendly way. Reusable form element.
This is developer module. Install it only when required by other module.
Modules that use Hierarchical Select Field Selector: Imagefield Avatar
This module provides HS implementation ready to use. To insert HS field selector in your form, do something like:
$form['your_field_selector'] = hs_field_selector_form_element();
You can filter node types and fields using your own callbacks:
$params = array('nodetype_filter' => 'your callback', 'field_filter' => 'your callback');
$form['your_field_selector']['#config']['params'] = $params;
For example of usage see Imagefield Avatar module.
Also you can read HS API documentation to learn how you can tweak this form element further.