Descobrindo o User Agent do browser usando JavaScript

<script type="text/javascript">
var userAgent = navigator.userAgent.toLowerCase();
alert(userAgent);
</script>

<script type="text/javascript">
/*
 * De acordo com o browser, redirecionar para determinada página
 */
 
var userAgent = navigator.userAgent.toLowerCase();
 
// Devices and Operating Systems
var devices = new Array('nokia','iphone','blackberry','sony','lg',
'htc_tattoo','samsung','symbian','SymbianOS','elaine','palm',
'series60','windows ce','android','obigo','netfront',
'openwave','mobilexplorer','operamini');
 
var url_redirect = 'http://example.com';
 
function mobiDetect(userAgent, devices) {
    for(var i = 0; i < devices.length; i++) {
        if (userAgent.search(devices[i]) > 0) {
               return true;
        }
    }
    return false;
}
 
if (mobiDetect(userAgent, devices)) {
    window.location.href = url_redirect;
}
 
// continua o código do site....
</script>