The Common Address Redundancy Protocol (CARP) was introduced by OpenBSD in October 2003 as a free and better replacement for VRRP. OpenBSD 3.5 (May 2004) was the first release to ship with CARP.
CARP allows to configure two or more hosts for failover and load-balancing. In conjunction with pf and pfsync it is possible to build a failsafe, stateful firewall without any configuration requirements for the clients. CARP is also a good fundation for a HA server cluster as it provides redundancy even while in load-balancing mode.
CARP was ported to FreeBSD shortly after it appeared in OpenBSD. It was committed to FreeBSD-CURRENT as of February 2005 and is going to be shipped with FreeBSD 5.4.
More information about what CARP can do for your network can be obtained from Ryan McBride's article Firewall Failover with pfsync and CARP.
Main FreeBSD PF page.