Saw this one in use a little while back, snagged it to review, and hadn't gotten around to it yet.
Yeah I know "canned scripts"...but hey, it's a fast response  <img border=0 align=absmiddle src="http://www.ozones.com/forum/wink.gif">
code:
<!--
// Ultimate client-side JavaScript client sniff. 
// (C) Netscape Communications 1999.  Permission granted to reuse and distribute. 
// Revised 17 May 99 to add is_nav5up and is_ie5up (see below). 
// See [url=http://www.it97.de/JavaScript/JS_tutorial/bstat/navobj.html]http://www.it97.de/JavaScript/JS_tutorial/bstat/navobj.html[/url]  and 
// [url=http://www.it97.de/JavaScript/JS_tutorial/bstat/Browseraol.html]http://www.it97.de/JavaScript/JS_tutorial/bstat/Browseraol.html[/url]  
// for detailed lists of userAgent strings. 
//   This Script Detects the screensize
//   and will go to the relevant HTML 
//   document. 
//   This script is Copyright © 1999
//   To Quantum-X, and Liquid Pixel
//   Design. If you use this script
//   leave this message intact.
    var agt=navigator.userAgent.toLowerCase(); 
    // *** BROWSER VERSION *** 
    // Note: On IE5, these return 4, so use is_ie5up to detect IE5. 
    var is_major = parseInt(navigator.appVersion); 
    var is_minor = parseFloat(navigator.appVersion); 
    // Note: Opera and WebTV spoof Navigator.  We do strict client detection. 
    // If you want to allow spoofing, take out the tests for opera and webtv. 
    var is_nav  = ((agt.indexOf('mozilla')!=-1) && (agt.indexOf('spoofer')==-1) 
                && (agt.indexOf('compatible') == -1) && (agt.indexOf('opera')==-1) 
                && (agt.indexOf('webtv')==-1)); 
    var is_nav2 = (is_nav && (is_major == 2)); 
    var is_nav3 = (is_nav && (is_major == 3)); 
    var is_nav4 = (is_nav && (is_major == 4)); 
    var is_nav4up = (is_nav && (is_major >= 4)); 
    var is_navonly      = (is_nav && ((agt.indexOf(";nav") != -1) | | 
                          (agt.indexOf("; nav") != -1)) ); 
    var is_nav5 = (is_nav && (is_major == 5)); 
    var is_nav5up = (is_nav && (is_major >= 5)); 
    var is_ie   = (agt.indexOf("msie") != -1); 
    var is_ie3  = (is_ie && (is_major < 4)); 
    var is_ie4  = (is_ie && (is_major == 4) && (agt.indexOf("msie 5.0")==-1) ); 
    var is_ie4up  = (is_ie  && (is_major >= 4)); 
    var is_ie5  = (is_ie && (is_major == 4) && (agt.indexOf("msie 5.0")!=-1) ); 
    var is_ie5up  = (is_ie  && !is_ie3 && !is_ie4); 
    // KNOWN BUG: On AOL4, returns false if IE3 is embedded browser 
    // or if this is the first browser window opened.  Thus the 
    // variables is_aol, is_aol3, and is_aol4 aren't 100% reliable. 
    var is_aol   = (agt.indexOf("aol") != -1); 
    var is_aol3  = (is_aol && is_ie3); 
    var is_aol4  = (is_aol && is_ie4); 
    var is_opera = (agt.indexOf("opera") != -1); 
    var is_webtv = (agt.indexOf("webtv") != -1); 
if (is_nav5up)
{
 window.location.href="711989Script2_2.html" 
}
else if (is_nav4)
{
 window.location.href="711989Script2_2.html"
}
else if (is_ie4up)
{
 window.location.href="711989Script3_2.html"
}
else if (is_nav3 | | is_opera)
{
 window.location.href="711989Script4_2.html"
}
else 
{
 window.location.href="711989Script4_2.html"
}
// -->