Fake: Redundant Server Switch
Fake has been designed to switch in backup servers on a LAN.

Linux Virtual Server Project
A virtual server is a highly scalable and highly available server built on a cluster of real servers. The architecture of the cluster is transparent to end users, and the users see only a single virtual server.

Conditional Execution Based on Host Availability
The Answer Guy 32: Conditional Execution Based on Host Availability.

Jungo: GO-HotSwap
Add CompactPCI hot-swap capabilities to: Linux / Embedded Linux, Solaris, VxWorks, Windows9x, Windows2000, Windows NT, and Windows NT- Embedded.

Linux-HA Project
Root page for High-Availability Linux Project; goal: Provide a high-availability (clustering) solution for Linux which promotes reliability, availability, and serviceability (RAS) through a community development effort.

failoverd
Software to provide rudimentary failover capability for Linux

Linux High Availability HOWTO
Linux High Availability HOWTO

Process Monitor HOW-TO for Linux
Process Monitor HOW-TO for Linux, to monitor and restart important processes on your system.

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.

Lemuria.org - Heart Software
A basic framework for a high-availability implementation. It exchanges and updates the status of all nodes within the cluster, so that a crash can be immediately noticed and appropriate action be taken.

MOSIX
MOSIX for Linux is a cluster-enablement technology that provides transparent process migration between nodes in a cluster. Migration is pre-emptive and transaparent to the user; processes run remotely via distributed system calls.

Software-RAID HOWTO: High Availability RAID
High Availability RAID from Software RAID HOWTO.

A High-Availability Cluster for Linux
A High-Availability Cluster for Linux A Linux Journal Article by Phil Lewis.

H.A. Technical Solutions
They make reliable high availability, data replication and load balancing software solutions including H.A. Clusters, H.A. EchoStream and H.A. Fulcrum.