Selecting multiple from an html select element without using ctrl key

Here is a pure JS solution:

element.onmousedown= function(event) {
    //this == event.target
    event.preventDefault();
    var scroll_offset= this.parentElement.scrollTop;
    this.selected= !this.selected;
    this.parentElement.scrollTop= scroll_offset;
}
element.onmousemove= function(event) {
    event.preventDefault();
}

Look at the parent element(the select box) and record the vertical scroll offset before selecting/deselecting the option. Then reset it manually once you have performed the action.

The reasoning behind preventing default behavior for the mousemove event is because if you don't prevent it and you happen to click an option while moving the mouse, all options will become de-selected.

You can save the Element.scrollTop and set it at the end.

$("select").mousedown(function(e){
    e.preventDefault();

    var select = this;
    var scroll = select .scrollTop;

    e.target.selected = !e.target.selected;

    setTimeout(function(){select.scrollTop = scroll;}, 0);

    $(select ).focus();
}).mousemove(function(e){e.preventDefault()});

http://jsfiddle.net/UziTech/cjjg68dr/114/

Tony's answer makes the select arrows buggy, as they work only if you hold the mouse down.

I've combined a few solutions into this, and it works fine at least in Chrome and FF:

// multiple select: selectable without Control
$('select[multiple] option').on('mousedown', function(e) {
    var $this = $(this),
        that = this,
        scroll = that.parentElement.scrollTop;

    e.preventDefault();
    $this.prop('selected', !$this.prop('selected'));

    setTimeout(function() {
        that.parentElement.scrollTop = scroll;
    }, 0);

    return false;
});