ESP8266 is a tiny extremity cheap MCU with third-party library making it Arduino compatible. Features all the normal connections you would expect such as GPIO, SPI, I2C, I2C and UART to name a few, the biggest selling point is its size and build in WiFi support with full stack TCP / IP communications.

Great for those IoT projects, connected devices and internet / network projects.

Features

• 32-bit RISC CPU: Tensilica Xtensa L106 running at 80 MHz*
• 64 KiB of instruction RAM, 96 KiB of data RAM
• External QSPI flash: 512 KiB to 4 MiB* (up to 16 MiB is supported)
• IEEE 802.11 b/g/n Wi-Fi
  • Integrated TR switch, balun, LNA, power amplifier and matching network
  • WEP or WPA/WPA2 authentication, or open networks
• 16 GPIO pins
• SPI
• I²C
• I²S interfaces with DMA (sharing pins with GPIO)
• UART on dedicated pins, plus a transmit-only UART can be enabled on GPIO2
• 10-bit ADC (this is a Successive Approximation ADC)

More Information

https://en.wikipedia.org/wiki/ESP8266