draggable.js

$.fn.extend({

  draggable : function() {
    this.each(function() {

      var drag = _.bind(function(e) {
        e.stopPropagation() && e.preventDefault();
        this.style.left = this._drag.left + event.pageX - this._drag.x + 'px';
        this.style.top  = this._drag.top + event.pageY - this._drag.y + 'px';
      }, this);

      var dragEnd = _.bind(function(e) {
        e.stopPropagation() && e.preventDefault();
        $(document.body).unbind('mouseup', dragEnd);
        $(document.body).unbind('mousemove', drag);
        $(this).removeClass('dragging');
      }, this);

      var dragStart = _.bind(function(e) {
        e.stopPropagation() && e.preventDefault();
        $(this).addClass('dragging');
        this._drag = {
          left : parseInt(this.style.left, 10) || 0,
          top  : parseInt(this.style.top, 10) || 0,
          x    : e.pageX,
          y    : e.pageY
        };
        $(document.body).bind('mouseup', dragEnd);
        $(document.body).bind('mousemove', drag);
      }, this);

      $(this).bind('mousedown', dragStart);
    });
  }

});