Oracle Tablespace usage query

select tablespace_name, owner, sum(bytes)/1024/1024 Mb
 from dba_segments
 where owner not in ('SYS','SYSTEM')
 group by tablespace_name, owner
 order by 3

http://www.blacksheepnetworks.com/security/resources/www.think-forward.com/sql/du.htm

A good reference for sqlplus

http://www.psoug.org/reference/sqlplus.html

-Kalyan