net/pfflowd, collects PF log data, converts to Cisco NetFlow™ for further processing (also see flowd by the same developer) - independent of pflow support
SNMP snmpd in OpenBSD base or net/net-snmp, PF-Related SNMP MIBs from Joel Knight's site, http://www.packetmischief.ca/openbsd/snmp are getting integrated