The Heartbeat program is one of the core components of the Linux-HA (High-Availability Linux) project. Heartbeat is highly portable, and runs on every known Linux platform, and also on FreeBSD and Solaris. Ports to other OSes are also in progress.
Heartbeat is the first piece of software which was written for the Linux-HA project. It performs death-of-node detection, communications and cluster management in one process.