Custom Multiple-select Control

Allowing users to multiple-select items in a list is a common design requirement. Using check boxes on a page or screen is fine when there are only a few items to select, but when there are many items, the list can consume too much screen real estate.

This custom control tucks the check-box list into a drop-down menu.

From: ShopWiki

A challenge with this approach is giving the user feedback as to what has been selected. This control shows a partial list, which persists after the control is closed.

Control Open:

Control Closed:
 
One thing this control could do that it doesn't is use a rollover behavior to show all items selected, which would make the "closed" view more useful.