ZMQ or ZeroMQ is one of the coolest open source libraries out there. It fills a real necessity in a new, brilliant way. It is very active, has bindings to many languages… that’s why it is so popular.
I was a professor in university, teaching C/C++, software engineering, distributed systems (plain old sockets, SOAP, web…), and I really enjoyed keeping my classes up to date and teaching last trends and technologies. If I was still a teacher, I would definitely teach some sessions using ZeroMQ.