[Source]

בלי שפות צד שרת:

<script language="javascript">
  var rand = Math.floor(Math.random()*20) + 1;
  var url;
  switch (rand)
  {
    case 1: url = "http://google.com"; break;
    case 2: url = "http://hosts.com"; break;
    .
    .
    .
    case 20: url = "gmail.com"; break;
    default: url = "http://thisSite.com";
  }

  location.href = url;
</script>