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

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;