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.
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.