jQuery.fn.check = function(mode) {
   // if mode is undefined, use 'on' as default
   var mode = mode || 'on';
   
   return this.each(function() {
     switch(mode) {
       case 'on':
         this.checked = true;
         break;
       case 'off':
         this.checked = false;
         break;
       case 'toggle':
         this.checked = !this.checked;
         break;
     }
   });
 };