set linesize 121
---------------------------------------------------------------------------------------
-- free.sql
--
-- This SQL Plus script lists freespace by tablespace
----------------------------------------------------------------------------------------
column dummy noprint
column  pct_used format 999.9       heading "%|Used"
column  name    format a19      heading "Tablespace Name"
column  Kbytes   format 999,999,999    heading "KBytes"
column  used    format 999,999,999   heading "Used"
column  free    format 999,999,999  heading "Free"
column  largest    format 999,999,999  heading "Largest"
column  max_size format 999,999,999 heading "MaxPoss|Kbytes"
column  pct_max_used format 999.9       heading "%|Max|Used"
break   on report
compute sum of kbytes on report
compute sum of free on report
compute sum of used on report
                                                                                                         
select (select decode(extent_management,'LOCAL','*',' ') ||
               decode(segment_space_management,'AUTO','a ','m ')
          from dba_tablespaces where tablespace_name = b.tablespace_name) ||
nvl(b.tablespace_name,
             nvl(a.tablespace_name,'UNKOWN')) name,
       kbytes_alloc kbytes,
       kbytes_alloc-nvl(kbytes_free,0) used,
       nvl(kbytes_free,0) free,
       ((kbytes_alloc-nvl(kbytes_free,0))/
                          kbytes_alloc)*100 pct_used,
       nvl(largest,0) largest,
       nvl(kbytes_max,kbytes_alloc) Max_Size,
       decode( kbytes_max, 0, 0, (kbytes_alloc/kbytes_max)*100) pct_max_used
from ( select sum(bytes)/1024 Kbytes_free,
              max(bytes)/1024 largest,
              tablespace_name
       from  sys.dba_free_space
       group by tablespace_name ) a,
     ( select sum(bytes)/1024 Kbytes_alloc,
              sum(maxbytes)/1024 Kbytes_max,
              tablespace_name
       from sys.dba_data_files
       group by tablespace_name
       union all
      select sum(bytes)/1024 Kbytes_alloc,
              sum(maxbytes)/1024 Kbytes_max,
              tablespace_name
       from sys.dba_temp_files
       group by tablespace_name )b
where a.tablespace_name (+) = b.tablespace_name
order by 1
/

If you are stuck on software written last century: 

--------------------------------------------------------
-- free.sql
--
-- This SQL Plus script lists freespace by tablespace
--------------------------------------------------------
                                                                                                         
column dummy noprint
column  pct_used format 999.9       heading "%|Used"
column  name    format a19      heading "Tablespace Name"
column  Kbytes   format 999,999,999    heading "KBytes"
column  used    format 999,999,999   heading "Used"
column  free    format 999,999,999  heading "Free"
column  largest    format 999,999,999  heading "Largest"
column  max_size format 999,999,999 heading "MaxPoss|Kbytes"
column  pct_max_used format 999.9       heading "%|Max|Used"
break   on report
compute sum of kbytes on report
compute sum of free on report
compute sum of used on report
                                                                                                         
select (select decode(extent_management,'LOCAL','*',' ')
          from dba_tablespaces where tablespace_name = b.tablespace_name) ||
nvl(b.tablespace_name,
             nvl(a.tablespace_name,'UNKOWN')) name,
       kbytes_alloc kbytes,
       kbytes_alloc-nvl(kbytes_free,0) used,
       nvl(kbytes_free,0) free,
       ((kbytes_alloc-nvl(kbytes_free,0))/
                          kbytes_alloc)*100 pct_used,
       nvl(largest,0) largest,
       nvl(kbytes_max,kbytes_alloc) Max_Size,
       decode( kbytes_max, 0, 0, (kbytes_alloc/kbytes_max)*100) pct_max_used
from ( select sum(bytes)/1024 Kbytes_free,
              max(bytes)/1024 largest,
              tablespace_name
       from  sys.dba_free_space
       group by tablespace_name ) a,
     ( select sum(bytes)/1024 Kbytes_alloc,
              sum(maxbytes)/1024 Kbytes_max,
              tablespace_name
       from sys.dba_data_files
       group by tablespace_name
       union all
      select sum(bytes)/1024 Kbytes_alloc,
              sum(maxbytes)/1024 Kbytes_max,
              tablespace_name
       from sys.dba_temp_files
       group by tablespace_name )b
where a.tablespace_name (+) = b.tablespace_name
order by 1
/

执行后将输出如下各种信息：
Tablespace Name: 表空间名。如果以“*”开头，该表空间是 本地管理（locally managed），
如果以空格开头，则表空间是传统的数据字典（old-fashioned dictionary-managed）。 如果
第二个字母是“a”，该表空间是自动管理Automatic Segment Space Management (ASSM) ，
如果是“m”，则该表空间是手动管理 manually managed (pctused, freelists, and so on 
are used to control space utilization). 
Kbytes:该表空间所有数据文件的大小（KB）。 
Used: 已使用大小（KB）。 
Free: 未使用大小（KB）。 
% Used: 未使用比率（%）。 
Largest： 最大可利用连续块大小（KB）。(主要用于数据字典表空间)。 如果数据字典的连续块
大小比某些对象的next extent小，那么这些对象显示"out of space"错误信息，只有
有剩余空间时，才显示连续块的大小。
MaxPoss Kbytes： 最大扩展大小（KB）。 
% Max Used： 到目前为止已使用的最大扩展大小（KB）。