You might also want to look at the forks of ZeroMQ ... as well as CurveZMQ, protocol for secure messaging for ZeroMQ applications across the Internet which uses Daniel J. Bernstein's CurveCP security handshake as an encryption layer.

Especially noteworthy is the fork by Martin Sustrik (the main original developer responsible for the code behind ZeroMQ) who has started nanomsg as a POSIX-compliant rewrite of the ZeroMQ core library in C, rather than C++.