DRBD
Drbd is a block device which is designed to build high availability clusters. This is done by mirroring a whole block device via (a dedicated) network. You could see it as a network raid 1.

Kimberlite

Lifekeeper
SteelEye's LifeKeeper for Linux is a software application that ensures the continuous availability of applications by maintaining system uptime. LifeKeeper maintains the high availability of clustered Linux systems by monitoring system and application health, maintaining client connectivity and providing uninterrupted data access regardless of where clients reside - on the corporate Internet, intranet or extranet.

Linux FailSafe

Linux-HA
Linux High Availability Project

Linux Replicated High Availability Manager
I spend much of my time at work building, fix, and improving highly available UNIX systems, (mainly HP-UX and AIX platforms). Given that I have used Linux at home for over 7 years I was keen to look at building a similar environment for commodity Linux servers.

Linux Virtual Server Project

MC/Serviceguard (HP)
HP Serviceguard is a proven high availability clustering solution with over 100,000 licenses implemented worldwide. As an integrated virtualization component, customers have the flexibility to add other key technologies to a Serviceguard environment when their business demands it.

HA-OSCAR
High Availability Open Source Cluster Application Resource (HA-OSCAR) is an open source project that aims toward non-stop services in the HPC environment through a combined power of High Availability and Performance Computing solutions.

RSF-1
RSF-1's strength is in its "does exactly what is says on the tin" functionality which provides a very simple, but powerful, configuration and management framework that allows customers to deploy High-Availability solutions in a matter of hours rather than days or weeks and with a minimum of fuss.

ShaoLin HA Cluster

SRRD
Service Routing Redundency Daemon
SRRD implements a fully featured Cluster Server based soley on the OSPF routing protocol. SRRD is using a new OSPF feature named OSPF Opaque Link State Announcements as proposed in RFC 2370. These Opaque Link State Announcements allow to flood arbitrary data through an OSPF routing domain. SRRD is fully configurable over the web and supports SSL and PKI client and server authentication. It implements cluster server features like Service Groups, Service Dependencies as well as Critical Services. The participating cluster nodes may reside on different networks (LANs) accross the whole OSPF routing domain allowing fast failover between distant cluster nodes.

Veritas Cluster Server

WDX