All versions no grants:

  select TO_NUMBER(SUBSTR(dbms_session.unique_session_id,1,4),'XXXX') from dual;

10g and up

  select SYS_CONTEXT( 'USERENV', 'SID' )  from dual;
  select USERENV('SID') from dual;

All versions but need to grant select on v$mystat:

  select sid from v$mystat where rownum = 1 ;


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does a session have an open transaction

select s.sid from v$transaction t, v$session s where s.taddr=t.addr;

or

select DBMS_TRANSACTION.LOCAL_TRANSACTION_ID from dual;