Name: Eclipse Mosquitto™

Homepage: https://mosquitto.org/

Why should this be included in the repository?:
Its the most popular MQTT Broker.
If you're not aware, "MQTT is a machine-to-machine (M2M)/"Internet of Things" connectivity protocol."

I need it to communicate with my ESP8266 micro controller.
If you're not aware, ESP8266 is a micro controller that's cheaper, faster, smaller and generally a lot more capable than an arduino. It can literally run python (micropython).

Is it Open Source (yes/no): yes!

How many users do you anticipate will use this software?
It has around 1500 stars on github and it's developed by the Eclipse IDE guys, so I think a fair amount of people should find it useful.

Link to source tarball/zip file:
http://mosquitto.org/files/source/mosquitto-1.4.15.tar.gz

It's already present in most OS(es) repositories and is also available on Mac and Windows