Hi,
Was working on portal in SharePoint 2013, and had to validate few things by signing as a different user. Then realized that the option of “Sign in as Different user” was missing.
Followed these article to get past this issue
http://support.microsoft.com/kb/2752600
http://nickgrattan.wordpress.com/2012/07/23/sign-in-as-different-user-and-sharepoint-2013/
Hope it helps