The multi-select box is a standard widget for webforms. Yes, despite its wide usage, it's also one of the most difficult widgets to use. To select multiple items requires holding down the Control key or the Command key, depending on which system the user is on. if the key is not held for even one selection, the whole widget is reset.
After a client had a very long day of re-selecting items, they asked for the select widget to be replaced with a colleciton of checkboxes.