Creating dynamic tabs is a powerful way to provide access to specific resources within the portal. For example, you can direct the portal users to view their Account record (associated with their Contact in your CRM):
Create a Link Tab. Refer to the Help Document on how to create link tabs.
Name the tab "My Account"
Enter "/{!$User.AccountId}" as the Tab Link.
In the above example, $User refers to the current user where {!User.Name} would refer to a User record.