<select id="test" multiple="multiple" size="5">
<option>Yopyop</option>
<option>Yopyop</option>
<option>Yopyop</option>
<option>Yopyop</option>
<option>Yopyop</option>
</select>

<script>
window.onload=function() {
  multiple("test");
};

function multiple(el) {
  if(typeof el=="string") { el = document.getElementById(el); }
  var opts = [];
  for(var i=0;i<el.options.length;i++) {
    opts.push(el.options[i].selected);
    el.options[i].onclick = (function(k) {
      return function() {
        opts[k] = !opts[k];
        for(var i=0;i<this.parentNode.options.length;i++) {
          this.parentNode.options[i].selected = opts[i];
        }
      };
    })(i);
  }
}
</script>

Citation : Golmote

Tain ton deuxième paragraphe est tellement peu français que j'ai cru au premier abord qu'il s'agissait d'un genre de rébus...

window.onload=function() {
  multiple("test");
};

function multiple(el) {
	if (typeof el=="string") { el = document.getElementById(el); }
	var opts = [];
	
	for (var i = 0; i < el.options.length; i++) {
		el.options[i].selected = false;
		opts[i] = false;
	}
	
	el.onmouseup = function() {
		for (var i = 0; i < this.options.length; i++) {
			this.options[i].selected = (opts[i] && !this.options[i].selected) || (!opts[i] && this.options[i].selected);
			opts[i] = this.options[i].selected;
		}
	};
}

<input type="checkbox">