cisco pix:
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_61/config/failover.htm
openbsd with pfsync + CARP:
http://www.countersiege.com/doc/pfsync-carp/
openbsd with pfsync, but not CARP:
http://www.seattlecentral.edu/~dmartin/docs/bridge.html
(CARP doesn’t work with bridges)