In a training course I’m giving at the moment, we’ve just gone over a few of the lesser known uses, along with the widely used ones:

• The obvious: ordering by a column name

  
  ORDER BY advance
  
  

• Less obvious: ordering by an expression on a column

  
  ORDER BY id % 2 -- even IDs first, then odd
  
  

• Ordering by multiple fields

  
  ORDER BY id % 2 DESC, price 
         -- odd orders first by even/odd IDs, then by prices within each
  
  

• Ordering by visible column position

  
  SELECT id, customerid, price * quantity AS "gross revenue"
  FROM orders
  ORDER BY 2
  
  

• Ordering by column aliases

  
  SELECT id, customerid, price * quantity AS "gross revenue"
  FROM orders
  ORDER BY "gross revenue"
  
  

The last one is particularly interesting, because column aliases can’t be used in other places e.g. in the where clause or from clause.