Zeromq2-uml is a project mainly written in ..., it's free.
UML diagrams for ZeroMQ2
Although ZeroMQ has a simple API, the internals are quite complex. This project aims to demystify the internal structure of ZeroMQ. It should be very helpful for anyone who wants to start contributing to ZeroMQ.
ZeroMQ Website: http://www.zeromq.org/ ZeroMQ architecture: http://www.zeromq.org/whitepapers:architecture
Git repository: http://github.com/zeromq/zeromq2
DIA UML editor website: http://live.gnome.org/Dia
Free use of this software is granted under the terms of the GNU Lesser General Public License (LGPL).