The Linux-HA (High-Availability Linux) project provides a high-availability (clustering) solution for Linux, FreeBSD, OpenBSD, Solaris and Mac OS X which promotes reliability, availability, and serviceability (RAS).
The project’s main software product is Heartbeat, a GPL-licensed portable cluster management program for high-availability clustering. Its most important features are:

no fixed maximum number of nodes - Heartbeat can be used to build large clusters as well as very simple ones
resource monitoring: resources can be automatically restarted or moved to another node on failure
fencing mechanism to remove failed nodes from the cluster
sophisticated policy-based resource management, …