Just some quick notes for later expansion
The Linux High-Availability project at www.linux-ha.org is the resource for [[wp:Linux-HA|High Availability]] with free software. [[wp:Failover|Failover]] is the part of a Highly Available system when primary resources automatically switch to reserve or stand-by resources.
See their [http://www.linux-ha.org/wiki/Downloads downloads page] for '''Heartbeat''', '''Cluster Glue''' and '''Resource Agents'''