Jquery Validate spanish DNI (adapted from http://xandrusoft.blogspot.com.es/2009/08/validar-dni-en-un-formulario.html)

dnivalidate.js

$.validator.addMethod("dniCheck", function(value, element) {
  if(/^([0-9]{8})*[a-zA-Z]+$/.test(value)){
    var numero = value.substr(0,value.length-1);
    var let = value.substr(value.length-1,1).toUpperCase();
    numero = numero % 23;
    var letra='TRWAGMYFPDXBNJZSQVHLCKET';
    letra = letra.substring(numero,numero+1);
    if (letra==let) return true;
    return false;
  }
  return this.optional(element);
}, "DNI no válido");
$('FORM').validate({
  rules: {
    'FORMFIELDNAME': { dniCheck: true }
  }
});