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.
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.
OpenAMQ is a fast, stable, free implementation of the AMQP (www.amqp.org) messaging protocol. It gives applications high-performance messaging (delivery of opaque data) across loosely connected networks. It is capable of handling up to 600k messages per second depending on the hardware, with a latency around 300 microseconds. It is multithreaded and supports C/C++ and Java on Linux, AIX, Solaris, OS/X, Windows, and OpenVMS.