Find Contacts who are in one and only one group?
Since sql is probably your best bet, the sql is pretty straight forward.  

SELECT c.display_name, 
       g.name, 
       count(*)
FROM   civicrm_group_contact gc,
       civicrm_contact c,
       civicrm_group g
where  gc.contact_id = c.id
and    gc.group_id = g.id
and    gc.status = 'Added'
group by 1
having count(*) = 1