http://man.openbsd.org/pfsync has more details. This is interesting, I also didn't know of its existence. That said, I've never had to administer multiple failover firewalls so my interest is purely out of curiosity.
"carp" should also be included in discoveries alongside pfsync. carp functions like cisco/juniper/etc VRRP to enable real-time failover. With pfsync being a stateful firewall a lot of care needs to happen with failovers. carp + pfsync allow you to run multiple hot pfsync firewalls that have synchronized state and have near-instant failover without state-related hiccups.
We use pf+pfsync+carp extensively over in FreeBSD.org as well. It's good stuff!
We use pf+pfsync+carp extensively over in FreeBSD.org as well. It's good stuff!