For me, the distributed lock manager, queues (SYS$ENC / SYS$DEQ), IPC via message passing (mailboxes), SYS$QIO to literally queue IO requests, event flags (SYS$SETEF / SYS$WAITFR)? Amazing clustering, - UNIX just seemed so backwards after learning VMS first! (Apologies though if I got any of those system call names wrong, its been a long time!)