Common Address Redundancy Protocol (CARP)
Introduced with OpenBSD 3.5
Patent free alternative to VRRP (RFC 2281, 3768, patent owners: Cisco, IBM, Nokia)
Firewall/server redundancy
Virtual network interface for automatic failover
pfsync
Virtual network interface (assigned to physical interface)
Handles syncronization between PF firewalls (in advance of failover)
Best reference: http://www.countersiege.com/doc/pfsync-carp/