http://harvesthq.github.com/chosen/

Great tool for enhancing selects, multiple selects, grouped selects etc. made by the Harvest team (great timetracking tool we're using on a daily basis). We're all about using native controls as much as possible, but multiple select boxes were always a non-usable pain in the ass. This one is a great solution in tight areas where you can't afford displaying a list with checkboxes.