field_default_form in modules/field/field.form.inc
Creates a form element for a field and can populate it with a default value.
field_multiple_value_form in modules/field/field.form.inc
Special handling to create form elements for multiple values.
image_field_widget_form in modules/image/image.field.inc
Implements hook_field_widget_form().
list_field_formatter_view in modules/field/modules/list/list.module
Implements hook_field_formatter_view().
number_field_widget_form in modules/field/modules/number/number.module
Implements hook_field_widget_form().