Salesforce: Formula for Owner

In Summer '13 release, Salesforce open Owner lookup field and made it available for cross-object formula. Owner field is special compare to other fields, because if the object is available for Queue (e.g. Case, Lead, and custom object), it can be linked to User object or to Group object (owner is a Queue).

You can access things like the record owner email address or owner manager name through formula field or workflow rule.

For example, if you need to get owner email address and it is not for queue, your formula would be Owner:User.Email, this formula field will return blank, if the object is available for Queue and record owned by a Queue.

From above sample, if the object supported by Queue, you can enhance the formula to:
IF( ISBLANK(Owner:User.Id), Owner:Queue.QueueEmail, Owner:User.Email )

To get User name or Queue name:
IF( ISBLANK(Owner:User.Id), Owner:Queue.Name, Owner:User.FirstName & " " & Owner:User.LastName )

User and Queue objects can be referenced on the Owner lookup, but must be explicitly selected, see screenshot below:

Reference: Cross-Object Owner Fields: A Powerful New Formula Option