It would be useful to have a more advanced multiselect than the standard one provided by HTML. For an example, see http://verens.com/archives/2005/04/27/son-of-multiselect/

The normal HTML "select multi" is very impractical for a wiki: Many 'normal' users don't immediately understand that you have to hold down Ctrl, or simply overlook it, thereby deselecting everything that others had selected.

From TWiki:Codev.ExtendingFormControls I gather that the logic to add such functionality is already in place, so it should be relatively "easy" to integrate son-of-multiselect or similar code. Unfortunately I am not very familiar with either Perl or Javascript...