Acid Media
Renders a scrollable list from HTML selects for easy data picking on touch devices. It comes with theming capability, optional jQuery Mobile integration and multiple display modes: Inline, Modal, Bubble, Docked Top & Bottom. It works with optgroups as well.
James Ducker
Selectify is a UI overlay for select elements which does not screw up your events.
Brian Reavis
Selectize is a jQuery-based custom <select> UI control. Useful for tagging, contact lists, country selectors, etc.
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.
Fast, simple and light jQuery plugin to customize HTML selects
John Tabernik
jQuery plugin for maintaining a list of options with two list boxes. This is an early beta--comments are welcome.
Per Vestman
Yet another jQuery plugin to style/replace select elements.
Mika Tuupola
Simple chained selects. You can choose from two different versions. Normal version uses uses classnames of select options to decide content of child select. It does not make any external AJAX(J) queries. Remote version makes external query and builds the child select from returned JSON response.
P'unk Avenue
Selective provides multiple selection with autocomplete: a list to which items may be added by typing part of the label. A server-side or client-side source may be specified for autocomplete (see jquery.ui.autocomplete). Items may be reordered if the sortable option is present (this feature requires jQuery UI sortable). Items may also be removed. There are additional features for advanced cases, such as adding options on the fly (for instance, for tagging, or describing attributes of a relationship, like the job title of an employee in a particular department). It is especially useful when there are too many options to be presented in an ordinary dropdown list.
Ben Plum
A jQuery plugin for replacing default select elements. Part of the Formstone Library.