PL/SQL Packages DBMS_SPACE

Analyze segment growth and space requirements.

Subprocedures:

UNUSED_SPACE Information about unused space in an object
             (table, index, or cluster). 

FREE_BLOCKS  Information about free blocks in an object
             (table, index, or cluster). 

SPACE_USAGE  Information about free blocks in a bitmapped segment. 
 

For full documentation of the packaged procedures above see the Oracle Manual:
"Oracle9i Supplied PL/SQL Packages and Types Reference"

or the book Oracle Built in Packages by Steven Feuerstein et al