Transactions and undo related things

Show active transactions

select t.start_time, s.inst_id, s.sid, s.serial#, s.username, s.osuser, s.machine, s.program, s.logon_time, s.last_call_et, s.wait_class, s.seconds_in_wait
  from gv$transaction t
     , gv$session s
 where s.inst_id = t.inst_id
   and s.saddr = t.ses_addr;

Show minimum active scn (used eg by storage indexes)

select min_act_scn from x$ktumascn;