ESP8266 - parasquid/mcu-iot-sensors GitHub Wiki
Firmware
Original AT firmware can be downloaded from espressif: https://www.espressif.com/en/support/download/at (I personally use 1.6.2)
There's also a good writeup of the the MCU's capabilities: https://tttapa.github.io/ESP8266/Chap04%20-%20Microcontroller.html
NodeMCUv2
https://www.seeedstudio.com/NodeMCU-v2-Lua-based-ESP8266-development-kit.html
Built-in led:
D4
I2C
SDA=4 => D2
SCL=5 => D1
SPI
M-CLK => D5
MISO => D6
MOSI => D7
SS CS => D8