harvest
Chosen is a JavaScript plugin that makes long, unwieldy select boxes much more user-friendly. It is currently available in both jQuery and Prototype flavors.
Archer Hsieh
A jQuery plugin that is a shuttle box ui.
Tamer Aydin
A plugin to present multiple select elements in easier way.
Igor Vaynberg
Select2 is a jQuery based replacement for select boxes. It supports searching, remote data sets, and infinite scrolling of results.
lou
This is a user-friendlier drop-in replacement for the standard <select> with multiple attribute activated.
Nicolas Bize
MagicSuggest is a multiple selection auto suggest combo box for Bootstrap 3.
Ilya Kremer
A combobox plugin