DBA_FREE_SPACE

Free extents in all tablespaces

Columns
   ___________________________
 
   TABLESPACE_NAME
      Name of the tablespace containing the extent
   FILE_ID
      ID number of the file containing the extent
   BLOCK_ID
      Starting block number of the extent 
   BYTES
      Size of the extent in bytes
   BLOCKS
      Size of the extent in ORACLE blocks
   RELATIVE_FNO
      Relative number of the file containing the extent

Example

--Free space in each tablespace:

COLUMN tablespace_name FORMAT A15
COLUMN "Total Free (MB) " FORMAT 999,999,999,999
COLUMN "Largest Free Extent (MB) " FORMAT 999,999,999,999

Select tablespace_name,
Sum(bytes/(1024*1024)) "Total Free (MB) ",
Max(bytes/(1024*1024)) "Largest Free Extent (MB) "
From dba_free_space
Group By tablespace_name;