ImpersonationServiceprovided by portal for impersonating.
But starting from Version 7.0, WPS ships a Out of box Impersonation portlet. This portlet is pretty simple, it lets you search for a user and then once you click on the name of the user it starts impersonating that user.
I just created a simple page and added Impersonation portlet to it like this, i want to impersonate "Sunil Patil" so i searched for it
Now when i click on the user name the impersonation session starts, which means the user is logged in as impersonated user which is "Sunil Patil" in my case and it takes user to Home page like this
I will have to logout from the portal before i go back to the impersonation portlet, if i try going to the impersonation portlet i get this error