ESP8266

It was first sold as a wireless modem: the ESP-01 module hung off another microcontroller's UART and took AT commands. The documentation was mostly in Chinese. Hobbyists soon worked out that the chip itself had a 32-bit Tensilica core at 80 MHz — more powerful than the Arduino it was supposed to serve — and started writing firmware for it directly.

Within a year there was NodeMCU running Lua and an Arduino port, and the modem had become the main processor. Flash sits outside on SPI, RAM is tight, and the Wi-Fi stack expects control back often; hog the CPU and the watchdog bites. The ESP32 replaced it in 2016, but the ESP8266 lives on in millions of smart plugs.