Tasmota

Tasmota is open-source firmware for devices built on ESP8266 and ESP32 chips, such as many smart plugs, switches and sensors from Sonoff and Tuya. You flash it onto compatible hardware to replace the vendor's firmware, which gives you local control rather than dependence on a manufacturer's cloud.

Configuration happens through a web interface, and updates can be delivered over the air. Automation is available through timers and rules, and the device can be controlled locally over MQTT, HTTP, serial or KNX. It talks to your own MQTT broker or a cloud one over TLS, including AWS IoT Core and Azure IoT Hub, or can run standalone with no server at all. Integrations include Home Assistant, Domoticz, openHAB and Node-RED. The code is written in C for PlatformIO and covers the ESP32 family, such as ESP32-S2, S3, C3, C5, C6 and P4.

Tasmota is licensed under GPL-3.0 and has extensive online documentation. It is a favorite of smart-home enthusiasts who want inexpensive devices that work offline and can be fully customized. Because it is firmware, it needs compatible hardware and some willingness to flash devices, so it suits hobbyists and installers more than casual users.