Useful scripts for DBA perspective -

http://www.dbapool.com/scripts.php

More stuff on finding free space available in a database -

http://forums.oracle.com/forums/thread.jspa?threadID=624042

A sql that we found useful is here - (from the above link itself) -

SELECT
NVL (b.tablespace_name, NVL (a.tablespace_name, 'UNKOWN')) NAME ,
mbytes_alloc mbytes ,
mbytes_alloc - NVL (mbytes_free, 0) used ,
NVL (mbytes_free, 0) free ,
((mbytes_alloc - NVL (mbytes_free, 0)) / mbytes_alloc) * 100 pct_used,
100 - (((mbytes_alloc - NVL (mbytes_free, 0)) / mbytes_alloc) * 100) pct_free
FROM
(
SELECT
SUM(BYTES) / 1024 / 1024 mbytes_free,
tablespace_name
FROM
SYS.dba_free_space
GROUP BY
tablespace_name
)
a,
(
SELECT
SUM(BYTES) / 1024 / 1024 mbytes_alloc,
tablespace_name
FROM
SYS.dba_data_files
GROUP BY
tablespace_name
)
b
WHERE
a.tablespace_name(+) = b.tablespace_name

UNION ALL

SELECT
f.tablespace_name ,
SUM (ROUND((f.bytes_free + f.bytes_used) / 1024 / 1024, 2) ) "total MB" ,
SUM (ROUND(NVL (p.bytes_used, 0) / 1024 / 1024, 2)) "Used MB" ,
SUM (ROUND ( ((f.bytes_free + f.bytes_used) - NVL (p.bytes_used, 0) ) / 1024 / 1024, 2 ) ) "Free MB" ,
(SUM (ROUND (NVL (p.bytes_used, 0) / 1024 / 1024, 2)) * 100) / (SUM (ROUND ((f.bytes_free + f.bytes_used) / 1024 / 1024, 2))),
100 - (SUM (ROUND (NVL (p.bytes_used, 0) / 1024 / 1024, 2)) * 100) / (SUM (ROUND ((f.bytes_free + f.bytes_used) / 1024 / 1024, 2)))
FROM
SYS.v_$temp_space_header f,
dba_temp_files d ,
SYS.v_$temp_extent_pool p
WHERE
f.tablespace_name(+) = d.tablespace_name AND
f.file_id(+) = d.file_id AND
p.file_id(+) = d.file_id
GROUP BY
f.tablespace_name
ORDER BY
5 DESC --&orderby
;