UPDATE May 2013; I just noticed that the KB article has been changed by Microsoft; they are now saying use the Run as another user option to start IE. So, I have updated my post to provide you the previous solution.
If you are working with SharePoint 2007 or 2010, you already know that you have the ability to logon as another user to a SharePoint site after being already authenticated with your account
This option may be helpful to change the user context on SharePoint (even if I’m not really recommend it; I prefer to start IE with “run as” command).
With SharePoint 2013, this option has disappear
Forget about this KB
Microsoft has published a KB to explain how to workaround the missing ‘”Sign-in as Different user” feature http://support.microsoft.com/kb/2752600
Edit the Welcome.ascx file, located within the C:\Program Files\Common Files\Microsoft Shared\Web Server Extension\15\TEMPLATE\CONTROLTEMPLATES directory with the following code – the code must be added before the existing element with the id of “ID_RequestAccess”
<SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser"
Hopefully, we may expect this to be solved with an update.
After applying the workaround (no need to perform an IISRESET)