How to list active / open connections in Oracle?

Question

Is there any hidden table, system variable or something to show active connections in a given moment?

Answer 1

When I'd like to view incoming connections from our application servers to the database I use the following command:

SELECT username FROM v$session 
WHERE username IS NOT NULL 
ORDER BY username ASC;

Simple, but effective.

Answer 2

For a more complete answer see: http://dbaforums.org/oracle/index.php?showtopic=16834

select
       substr(a.spid,1,9) pid,
       substr(b.sid,1,5) sid,
       substr(b.serial#,1,5) ser#,
       substr(b.machine,1,6) box,
       substr(b.username,1,10) username,
--       b.server,
       substr(b.osuser,1,8) os_user,
       substr(b.program,1,30) program
from v$session b, v$process a
where
b.paddr = a.addr
and type='USER'
order by spid; 

Answer 3

select 
    count(1) "NO. Of DB Users", 
    to_char(sysdate,'DD-MON-YYYY:HH24:MI:SS') sys_time
from 
    v$session 
where 
    username is NOT  NULL;