Oracle Login Context Information

Find the Database instance you are logged into
SELECT sys_context('USERENV', 'DB_NAME') FROM dual;
SELECT sys_context('USERENV', 'INSTANCE_NAME') FROM dual;

Find the operating system user hosting the Oracle session
SELECT sys_context('USERENV', 'OS_USER') FROM dual;

Find the Oracle user currently logged into
SELECT sys_context('USERENV', 'SESSION_USER') FROM dual;

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

Oracle Space queries

Get Used space for current user
SELECT sum(bytes)/(1024 * 1024) as "Used (MB)"
FROM user_SEGMENTS
ORDER BY 1 desc

Get tablespace usage for all users using that tablespace
select owner, sum(bytes)/power(2,20)mb
from dba_extents
where tablespace_name = 'TABLESPACE'
group by owner
order by 2 desc;

Get Total tablespace available for current user
select tablespace_name,round(sum(bytes) / (1024 * 1024),2) "Tablespace SIZE (MB)"
from user_free_space
group by tablespace_name

Get space of individual tables/indexes (objects) in a schema
select sum(bytes) / (1024 * 1024) as MB,owner,segment_type, segment_name
from dba_segments s
where owner = 'SCHEMA'
group by owner,segment_type, segment_name
order by MB desc

Get USER_ dictornary Tables
SELECT table_name, comments
FROM dictionary
WHERE table_name LIKE 'USER_%'
ORDER BY table_name;

http://snipplr.com/view/4748/get-a-list-of-all-the-user-tables-oracle/
http://www.freelists.org/post/oracle-l/dba-extents-vs-dba-segments,8