- The socket library that acts as a concurrency framework. - Faster than TCP, for clustered products and supercomputing. - Carries messages across inproc, IPC, TCP, and multicast. - Connect N-to-N via fanout, pubsub, pipeline, request-reply. - Asynch I/O for scalable multicore message-passing apps. - Large and active open source community. - 30+ languages including C, C++, Java, .NET, Python. - Most OSes including Linux, Windows, OS X. - LGPL free software with full commercial support.