if(navigator.appName == 'Microsoft Internet Explorer'){
    // load jquery
} else {
    // load zepto
}

$.os.ios      // => true if running on Apple iOS
$.os.android  // => true if running on Android
$.os.webos    // => true if running on HP/Palm WebOS
$.os.touchpad // => true if running on a HP TouchPad
$.os.version  // => string with version number, "4.0", "3.1.1", "2.1", etc.
$.os.iphone   // => true if running on iPhone
$.os.ipad     // => true if running on iPad
$.os.blackberry // => true if running on BlackBerry

var isSupported = false;
for (os in $.os) {
    if ($.os[os] == true) { isSupported = true; }
}

yepnope({
  test: !+"v1", // IE?
  yep: 'jquery.js',
  nope: 'zepto.js'
});

<!--[if lt IE 8 ]>
    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.js"></script>
<![endif]-->

<!--[if !IE]>
    <script src="/js/zepto.js"></script>
<![endif]-->

if ('__proto__' in {}) {
    // IS NOT IE

  } else {
    // IS IE

  }

<script>
document.write('<script src=' +
('__proto__' in {} ? 'zepto' : 'jquery') +
'.js></script>')
</script>

<script>
if ('__proto__' in {}) {
  // This is NOT IE

  } else {
    // This is IE

  }
</script

<script>
  document.write('<script src=' + ('__proto__' in {} ? 'zepto' : 'jquery') + '.js></script>')
</script>

var isHtml5AndNotIE     = function() {
        var elem = document.createElement('canvas');
        return '__proto__' in {} && !!(elem.getContext && elem.getContext('2d'));
    };

<script type="text/javascript">
if(top.execScript){ // true only in IE
    document.write("<script src='/js/jquery.js'>x3C/script>");
}
else{
    document.write("<script src='/js/zepto.min.js'>x3C/script>");
}
</script>