Espressif ESP8266 and ESP32 are a series of low-cost, low-power system on a chip microcontrollers with integrated Wi-Fi and dual-mode Bluetooth. These small modules allow microcontrollers and platforms like the Arduino to connect to a Wi-Fi network and make simple TCP/IP connections. You can even use them like an Arduino, and take full advantage of their powerful computing and networking abilities.
The boards below incorporate an ESP8266 or ESP32 chip. There are plenty of tutorials and beginners guides available on the web.