A loadbalancer for other services. Utilizing Pound [1] and KeepaliveD [2] (note that keepalive is something totally different). For a sample config see [3]
[1] Pound: http://www.apsis.ch/pound [2] keepalived http://www.keepalived.org [3] howto http://www.howtoforge.com/high-availability-load-balancer-pound-keepalived-debian-etch