Carp/ucarp are pretty fun. Additionally keepalived in linux implements vrrp and some additional nice features such as scripting what to do on failover of resources.

An addendum to this guide might be to add in connection tracking across the master/slave nodes. In *BSD this is implemented with pfsync. In linux it would be iptables connection tracking.

More on pfsync: http://www.openbsd.org/faq/pf/carp.html#pfsyncop Linux conntrackd: http://conntrack-tools.netfilter.org/

Edit: Oh, and everyone always forgets about using LVS for load-balancing and failover. There's endless documentation on the web about that, and it's not a proxy service like haproxy (which is both good and bad).