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