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You can use userinfo class in apex to obtain the information related to logged in user.

https://www.salesforce.com/us/developer/docs/apexcode/Content/apex_methods_system_userinfo.htm

You can get Id using userinfo.getUserId() and if userinfo class is not able to provide info use query to query those fields .

On VF you will need to use

 <apex:outputText value="{!$User.Id}">

You are using outputfield and hence you are not able to save the code

Edit::

A sample getter setter User object type variable will be handy

public class MailExtension{
  public user currentuser{get;set;}
  public MailExtension(ApexPages.StandardController cont){
     opp =(Opportunity) cont.getRecord();
     currentuser=new User();
     currentuser=[Select Id,Name,Email from User where Id=:userinfo.getuserId()];
  } 
}

A sample VF tag or mark up will be as follows

<apex:outputfield value="{!currentuser.Id}"/>
<apex:outputfield value="{!currentuser.Email}"/>