All classes that implement WidgetInterface

class \Drupal\email\Plugin\field\widget\EmailDefaultWidget
class \Drupal\field\Plugin\Type\Widget\WidgetBase implements WidgetInterface
class \Drupal\field\Plugin\field\widget\LegacyWidget
class \Drupal\field_test\Plugin\field\widget\TestFieldWidget
class \Drupal\field_test\Plugin\field\widget\TestFieldWidgetMultiple
class \Drupal\field_test\Plugin\field\widget\TestFieldWidgetNoDefault
class \Drupal\file\Plugin\field\widget\FileWidget
class \Drupal\image\Plugin\field\widget\ImageWidget
class \Drupal\number\Plugin\field\widget\NumberWidget
class \Drupal\taxonomy\Plugin\field\widget\TaxonomyAutocompleteWidget
class \Drupal\text\Plugin\field\widget\TextareaWidget
class \Drupal\text\Plugin\field\widget\TextareaWithSummaryWidget
class \Drupal\text\Plugin\field\widget\TextfieldWidget