Multiselect

The Multiselect module defines a field widget to be used with the following fields types: List fields (text, float, or integer) or entity reference fields. It allows the user to select multiple items in an easy way. It consists of two lists, one of all available items, the other of selected items. The user can select an item by moving it from the unselected list to the selected list. This module relies on the built-in jQuery features of Drupal and does not require any additional modules.

Drupal 7:

• FAPI integration - allows module developers to create new form components using '#type' => 'multiselect'

Drupal 8:

A new release is meant to address some of the issues that have been lingering in the issue queue, including for FAPI and AJAX support.