A progressive enhancement is applied to the select multiple that makes it much easier to use. This enhancement automatically hides the original select multiple, and instead presents a regular select showing the available options, and an HTML list showing the already-selected options. While hidden, the original select multiple is updated as the user makes changes.
No Ctrl/Cmd-Clicking is necessary to select multiple items, and an accidental click is not destructive. You add or remove items one at a time. Only the already-selected items occupy screen real estate, as they should. A remove link is included with each selected item, which eliminates ambiguity. Viewing and choosing a new item to add requires clicking on the select. This action is inherently more familiar, and displays many more available options at once, than a select multiple. More is seen at a glance and less scrolling (if any) is required to see all available options.