The HTML select element has received much criticism when used for multiple selection over its usability problems. Although it may be simple enough for experienced users, it must be explained in detail (repeatedly) to the less experienced users, who will always find its behavior to be unintuitive. Recently we have come up against this problem and are attempting to work around it.

For some useful background and some great alternate implementations of multiple-selection see Ryan Cramer's analysis and replacement ideas as well as a jQuery Simple Multi-Select plugin which offers an on-demand replacement for the select multiple elements.

The approach that I took was to attempt to leave the select element alone as much as possible, since it is widely recognized and understood by HTML veterans (and already integrated in huge amounts of legacy code), and make it a little more user-friendly by removing the requirement of holding Ctrl to select multiple options. With the modifications in place, clicking on an option will toggle that option between selected and un-selected, while holding Ctrl or Shift will result in the same behavior as before. The solution is written in JavaScript with jQuery (although there is no reason it couldn't be reworked to remove the dependency on jQuery) and was completed without any browser sniffing (only feature-detection).

The script can be found in the forge on the Improve Multiple Select project page. The script can be directly downloaded from the files section (I suggest against directly linking to it - since the location may change). As always, feedback, suggestions for improvement, bug reports, etc. are greatly appreciated.