If you have started using SharePoint 2013 Preview, you might have noticed that there is no option to login as a different user. There is an option to sign out, but how to log back in?
One can use the link which is available in SharePoint 2010
It works in SharePoint 2013 Preview same as SharePoint 2010.
Another option is to use "Run as" while launching your browser.
I believe it is a handy feature to have for developers or administrators to login as different user.