While going through some of the examples in the Microsoft .NET Framework 3.5 – ASP.NET Application Development Self-Paced Training Kit, I came accross a nice Ajax component for evaluating the strength of password.

Add a JS file to the scripts folder and add this code:

///
Type.registerNamespace("AjaxEnabled");

//create constructor
AjaxEnabled.PasswordStrengthComponent = function() {
    AjaxEnabled.PasswordStrengthComponent.initializeBase(this);
}

//define class
AjaxEnabled.PasswordStrengthComponent.prototype = {
  initialize: function() {
    //add custom initialization here
    AjaxEnabled.PasswordStrengthComponent.callBaseMethod(this, 'initialize');
  },

  returnPasswordStrength: function(password) {
    var strPass = new String(password.toString());
    if (strPass.length <  5) {
      return "Weak";
    }
    else {
      if (strPass.length < 9) {
        return "Medium";
      }
      else {
        return "Strong";
      }
    }
  },

  dispose: function() {
    //add custom dispose actions here
    AjaxEnabled.PasswordStrengthComponent.callBaseMethod(this, 'dispose');
  }
}

//register class as a Sys.Component
AjaxEnabled.PasswordStrengthComponent.registerClass(
  'AjaxEnabled.PasswordStrengthComponent', Sys.Component);

//notify script loaded
if (typeof(Sys) !== 'undefined') Sys.Application.notifyScriptLoaded();

Add reference to the JS in the page using the script manager:

      <asp:ScriptManager ID="ScriptManager1" runat="server">
        <Scripts>
          <asp:ScriptReference Path="~/AjaxComponent.js" />
        </Scripts>
      </asp:ScriptManager>

Wire up the OnKeypress() event:

      <script language="javascript" type="text/javascript">

          function _OnKeypress() {
              var checker = new AjaxEnabled.PasswordStrengthComponent();
              var pass = document.getElementById("txtPassword").value;
              var strength = checker.returnPasswordStrength(pass);
              document.getElementById("lblStrength").innerText = strength;
          }

      </script>