WIFI ESP8266 WiFi Serial Transceiver Wireless Module 1MB Flash ESP-01

You could add WiFi capability to your Arduino project by using this module.

This module integrates with the ESP8266 IC. It has integrated TCP/IP protocol stack. It also promotes a complete WiFi networking solution, making it possible to host an application or taking all WiFi networking functions from the different application processors.

ESP8266 also has a powerful on-board processing and storage capability, meaning you could use this module alone in creating a simple project.

You could integrate this with your MCU by connecting it using the UART(RX/TX) and the power supply (3.3V/GND) to make this work.