Description :

The SparkFun ESP8266 WiFi Shield is a development board for the ESP8266 WiFi SoC
It is compatible with Arduino and finds a middle ground between the ESP8266 module and ESP8266 Thing
The board comes pre-flashed with AT-command firmware for control through any UART
It has a variety of I/O that can be configured as digital inputs or outputs, including an ADC
The GPIO are broken out towards the top-left side of the shield
The shield can be reprogrammed through the programming port found on the top-right side, which utilizes the pinout of the FTDI Basic breakouts
The shield is populated with components but does not come with headers installed
The recommended header kit is the Arduino R3 Stackable Header Kit

Features:

Arduino R3 Layout
802.11 b/g/n
Wi-Fi Direct (P2P), soft-AP
Integrated TCP/IP protocol stack
Integrated TR switch, balun, LNA, power amplifier and matching network
Integrated PLLs, regulators, DCXO and power management units
Integrated low power 32-bit CPU could be used as application processor
+19.5dBm output power in 802.11b mode