Calculate column running total
Well, apparently this is it:

SELECT 
    *, 
    SUM(amount) OVER (
        ORDER BY tx_date DESC 
        ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
    ) AS running_balance
FROM transactions
ORDER BY tx_date DESC;


Output:

 id |      memo       | amount |  tx_date   | running_balance 
----+-----------------+--------+------------+-----------------
  4 | Wages           |  75000 | 2015-08-31 |           77000
  2 | Eating out      |  -1000 | 2015-08-19 |            2000
  3 | Gas             |  -7000 | 2015-08-19 |            3000
  1 | Initial balance |  10000 | 2015-08-17 |           10000