ZeroMQ – Relicense from LGPL3 and exceptions to MPL 2.0
Back in 2014, I was rebuilding the core of an event processing engine.  At the time, the decision was between Apache Kafka and rolling our own.  After investigating Zookeeper, we decided to roll our own and chose ZeroMQ as the messaging layer as our on-prem customers probably didn't want to own and manage Zookeeper.ZeroMQ was absolutely solid and stable; incredibly trouble free and the only issues we ran into were when IT teams didn't open the ports we documented in the configuration procedure.  (The resultant architecture actually looks a lot like Flink)In any case, ZeroMQ is a fantastic piece of technology that I feel like I don't see out in the wild quite enough.  It's dead simple and incredibly stable from my experience.