NATS.io is a simple, secure and high performance open source messaging system for cloud native applications, IoT messaging, and microservices architectures.

Tasmota is an open source firmware for ESP8266 based devices.
Total local control with quick setup and updates.
Control using MQTT, Web UI, HTTP or serial.
Automate using timers, rules or scripts.
Integration with home automation solutions.

PlatformIO is an open source ecosystem for IoT development
Cross-platform build system. Continuous and IDE integration. Arduino and MBED compatible