Chosen Select

This module provides form element and field widget based on Select2 jQuery plugin.

Chosen Select form element could be flexibly used on any form.
You should specify form element type in 'chosenselect'. Other properties of the element are pointed as select form element. You should specify property '#placeholder'(help text that is displayed in the empty field) if you are using multiple mode.
For example:

$form['color'] = array(
  '#title' => t('Select color'),
  '#type'    => 'chosenselect',
  '#multiple' => TRUE,
  '#options' => array(
    0 => t('Red'),
    1 => t('Yellow'),
    2 => t('Black'),
    3 => t('Blue'),
    4 => t('Green'),
  ),
  '#placeholder' => t('Search for a color'),
  '#description' => t('Select one of the colors.'),
);

A field widget for all kinds of lists. It can be used for these types of fields: List (integer), List (float), List (text), Term reference.