jQuery-dependent code to detect browser type and OS, and populate a hidden field named "user_agent". BrowserDetect code from http://snipplr.com/view.php?codeview&id=63137.

browser_detect.html

<script>
  (function () {

    var BrowserDetect = {
      init: function () {
        this.browser = this.searchString(this.dataBrowser) || "An unknown browser";
        this.version = this.searchVersion(navigator.userAgent) || this.searchVersion(navigator.appVersion) || "an unknown version";
        this.OS = this.searchString(this.dataOS) || "an unknown OS";
      },
      searchString: function (data) {
        for (var i = 0; i < data.length; i++) {
          var dataString = data[i].string;
          var dataProp = data[i].prop;
          this.versionSearchString = data[i].versionSearch || data[i].identity;
          if (dataString) {
            if (dataString.indexOf(data[i].subString) != -1) return data[i].identity;
          } else if (dataProp) return data[i].identity;
        }
      },
      searchVersion: function (dataString) {
        var index = dataString.indexOf(this.versionSearchString);
        if (index == -1) return;
        return parseFloat(dataString.substring(index + this.versionSearchString.length + 1));
      },
      dataBrowser: [{
        string: navigator.userAgent,
        subString: "Chrome",
        identity: "Chrome"
      }, {
        string: navigator.userAgent,
        subString: "OmniWeb",
        versionSearch: "OmniWeb/",
        identity: "OmniWeb"
      }, {
        string: navigator.vendor,
        subString: "Apple",
        identity: "Safari",
        versionSearch: "Version"
      }, {
        prop: window.opera,
        identity: "Opera"
      }, {
        string: navigator.vendor,
        subString: "iCab",
        identity: "iCab"
      }, {
        string: navigator.vendor,
        subString: "KDE",
        identity: "Konqueror"
      }, {
        string: navigator.userAgent,
        subString: "Firefox",
        identity: "Firefox"
      }, {
        string: navigator.vendor,
        subString: "Camino",
        identity: "Camino"
      }, { // for newer Netscapes (6+)
        string: navigator.userAgent,
        subString: "Netscape",
        identity: "Netscape"
      }, {
        string: navigator.userAgent,
        subString: "MSIE",
        identity: "IE",
        versionSearch: "MSIE"
      }, {
        string: navigator.userAgent,
        subString: "Gecko",
        identity: "Mozilla",
        versionSearch: "rv"
      }, { // for older Netscapes (4-)
        string: navigator.userAgent,
        subString: "Mozilla",
        identity: "Netscape",
        versionSearch: "Mozilla"
      }],
      dataOS: [{
        string: navigator.platform,
        subString: "Win",
        identity: "Windows"
      }, {
        string: navigator.platform,
        subString: "Mac",
        identity: "Mac"
      }, {
        string: navigator.userAgent,
        subString: "iPhone",
        identity: "iPhone/iPod"
      }, {
        string: navigator.platform,
        subString: "Linux",
        identity: "Linux"
      }]

    };

    BrowserDetect.init();

    window.$.client = {
      OS: BrowserDetect.OS,
      Browser: BrowserDetect.browser,
      Version: BrowserDetect.version
    };

  })();

  $().ready(function() {
    $('#user_agent').val([$.client.OS, $.client.Browser].join('/'));
  });
</script>