Free space in Oracle tablespaces

I occasionally want to know what tablespaces I have defined, how big they are and how much free space is available.

Thanks to Praveen at http://www.expertsharing.com/2008/02/26/calculate-size-of-tablespace-free-space-of-a-tablespace/ for providing the following query, which makes this information available easily.

set pages 999;
set lines 132;
SELECT *
FROM
( SELECT
   c.tablespace_name,
   ROUND(a.bytes/1048576,2)                    MB_Allocated,
   ROUND(b.bytes/1048576,2)                    MB_Free,
   ROUND((a.bytes-b.bytes)/1048576,2)          MB_Used,
   ROUND(b.bytes/a.bytes * 100,2)              tot_Pct_Free,
   ROUND((a.bytes-b.bytes)/a.bytes,2) * 100    tot_Pct_Used
 FROM
   ( SELECT
       tablespace_name,
       SUM(a.bytes) bytes
     FROM
       sys.DBA_DATA_FILES a
     GROUP BY
       tablespace_name
   ) a,
   ( SELECT
       a.tablespace_name,
       NVL(SUM(b.bytes),0) bytes
     FROM
       sys.DBA_DATA_FILES a,
       sys.DBA_FREE_SPACE b
     WHERE
       a.tablespace_name = b.tablespace_name (+)
       AND a.file_id = b.file_id (+)
     GROUP BY
       a.tablespace_name
   ) b,
   sys.DBA_TABLESPACES c
 WHERE
   a.tablespace_name = b.tablespace_name(+)
   AND a.tablespace_name = c.tablespace_name
 )
WHERE
 tot_Pct_Used >=0
ORDER BY
 tablespace_name;