The 0MQ lightweight messaging kernel is a library which extends the standard socket interfaces with features traditionally provided by specialized messaging middleware products. 0MQ sockets provide an abstraction of asynchronous message queues, multiple messaging patterns, message filtering (subscriptions), seamless access to multiple transport protocols, and more.
Apache Qpid is a messaging broker that implements the latest AMQP specification, providing transaction management, queuing, distribution, security, management, clustering, federation, heterogeneous multi-platform support, and much more. It is extremely fast and aims to be 100% AMQP Compliant.
Bonka's Queue is an HTTP-based message queue that relates to JMS or AMQP much in the same way that CouchDB relates to traditional DBMSes. That is to say that it tries to solve a different (much simpler) problem and avoid all the extra overhead associated with more "proper" solutions.
The Clacks Framework enables implementators to abstractly manage infrastructure entities, such as users, groups, machines and structure items. These are freely defineable and mappable to various backends like LDAP or MongoDB. Additionally it can handle joined clients, collect inventory data, and manage these clients in various ways. All information, API, and events are exposed on an AMQP message bus, and can be used by shell scripts, Web GUIs, etc.
Collage is a cross-platform C++ library for building heterogenous, distributed applications. Among other things, it is the cluster backend for the Equalizer parallel rendering framework. It provides an abstraction of different network connections, peer-to-peer messaging, node discovery, synchronization, and high-performance, object-oriented, versioned data distribution. It is designed for low-overhead multi-threaded execution, which allows applications to easily exploit multi-core architectures.
HornetQ is a multi-protocol, embeddable, high performance, clustered, asynchronous messaging system. HornetQ is an example of Message Oriented Middleware. It includes an extensive, easy-to-understand user-manual and quick-start guide and over 65 ready-to-run examples out of the box, demonstrating everything from simple JMS usage to complex clusters of servers and more exotic functionality. Its elegant POJO based design has minimal third party dependencies. You can run HornetQ as a stand-alone messaging broker, run it in integrated in your favorite JEE application server, or run it embedded inside your own application. Its journal provides high persistent messaging performance. Automatically switching into native mode when running on Linux, it uses asynchronous IO to provide persistent messaging rates that can saturate the write throughput of a disk. Its pluggable transport system uses JBoss Netty out of the box to provide high performance and scalability on the wire.