Pre-configured projects with source code are located in PlatformIO Examples repository.
Getting Started with PlatformIO - HIGHLY RECOMMENDED FOR BEGINNERS
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arendst/tasmota - an alternative firmware for ESP8266 with easy configuration using webUI, OTA updates, automation using timers or rules, expandability and entirely local control over MQTT, HTTP, Serial or KNX
MarlinFirmware/Marlin - an optimized firmware for RepRap 3D printers based on the Arduino platform
scottbez1/smartknob - a haptic input knob with software-defined endstops and virtual detents
esphome/esphome - a system to control your ESP8266/ESP32 by simple yet powerful configuration files and control them remotely through Home Automation systems
xoseperez/espurna - a home automation firmware for ESP8266-based devices
1technophile/OpenMQTTGateway - MQTT gateway for ESP8266, ESP32, Sonoff RF Bridge or Arduino with bidirectional 433mhz/315mhz/868mhz, Infrared communications, BLE, Bluetooth, beacons detection, mi flora, mi jia, LYWSD02, LYWSD03MMC, Mi Scale, TPMS, BBQ thermometer compatibility, SMS & LORA
cyberman54/ESP32-Paxcounter - Wifi & BLE driven passenger flow metering with cheap ESP32 boards.