Clustering is the idea of making several computers act as one for the purposes of either performance or reliability. The Clustering Daemon guarantees that a response will be serviced provided at least one cluster node is up. It runs as a userspace process and provides a large amount of flexibility.
Condor is a high throughput system, scheduling and providing large amounts of computational power over a long period of time. It provides the efficient use of a large variety of systems, from idle desktop workstations and dedicated clusters to grid systems all over the world, while its incredibly flexible configuration implements and maintains the machine owner's desired policy for the machine's availability.
ISPMan is a distributed system used to manage components of an ISP from a central management interface. Its written entirely in Perl, using an LDAP backend to manage DNS, Apache virtual hosts, Postfix, Cyrus, FTP, etc. It provides a central Web-based user interface for admins/helpdesk and a commandline interface to automate tasks or hook to other systems.
JavaGroups is a Java package for reliable group communication. It consists of 3 parts: The first part is a socket-like API for application development, providing methods for joining and leaving a group, sending multicast messages to members, and receiving notifications when members join/leave/crash. The second part is a protocol stack which implements reliable communication, and can be configured according to the QoS desired. New protocols can easily be added, and existing ones removed/replaced/modified. The third part is a set of building blocks which give the application/protocol programmer high-level abstractions (e.g. DistributedHashtable, derived from java.util.Hashtable, which is similar to Linda/JavaSpaces).
JBoss is an Open Source, standards-compliant, Enterprise JavaBeans application server implemented in pure Java. JBoss provides JBossServer, the basic EJB container and JMX infrastructure, JBossMQ for JMS messaging, JBossMail for mail, JBossTX for JTA/JTS transactions, JBossSX for JAAS based security, JBossCX for JCA connectivity, and JBossCMP for CMP persistence. It integrates with Tomcat Servlet/JSP container and Jetty Web server/servlet container, and enables you to mix and match these components through JMX by replacing any component you wish with a JMX-compliant implementation for the same APIs. The goal is to provide a full J2EE stack in the Free/Open Source software world.
The Linux Virtual Server Project is a project to cluster many real servers together into a highly available, high-performance virtual server. The LVS load balancer handles connections from clients and passes them on the the real servers (so-called Layer 4 switching) and can virtualize almost any TCP or UDP service, like HTTP, HTTPS, NNTP, FTP, DNS, ssh, POP3, IMAP4, SMTP, etc. It is fully transparent to the client accessing the virtual service.