Ai-Thinker ESP8266 Wi-Fi Module ESP-07S 4MB

This ESP-07S is a version of the ESP8266 module series with 4MB memory and has an uFL connector on board for an external Wi-Fi antenna. (this has to be purchased separately!)

This module offers you the possibility to connect your Arduino to Wi-Fi for a very low price or to use it as a stand-alone Arduino.

The ESP8266 module works with the normal UART port on your Arduino and can connect to networks, form your own network (hot spot) or both. The possibilities with this module are endless!

If you want to make a start (in Dutch), look here. In addition, you can find enough information at SeeedStudio, Instructables - ESP8266 Wi-Fi Temperature Logger, ESP8266 forum, Hackaday and Nurdspace.
The module can also receive an update (more functionality, more stable, etc.). These sites have good descriptions:   ESP8266: Reflash Dance! and BikeAlive.

This variant has 4MB memory and must be soldered. The pins on the sides are usable for general use, the pins on the bottom (6 pins on the bottom of the second picture) can partly be used to program the flash chip. (these do not offer extra GPIO pins)

Note: This module may be delivered without a pre-programmed firmware!

Check the tab "Downloads" for more information and/or downloads.