The ESP8266 WiFi Module with 1Mb flash memory supports 802.11 b/g/n and has an integrated TCP/IP stack allowing microcontrollers like Arduinos to talk over standard WiFI. The module is programmable or can be used to add WiFi capability to another microcontroller.
This is the module version of the same chip that is found in the SparkFun Thing and the SparkFun WiFi Shield for Arduino. This tiny module can be programmed to run standalone or used as a WiFi add-on to any project.
The module is so easy to use because it has the TCP/IP stack integrated and supports the same 802.11 b/g/n standard found in home and office WiFi networking gear. Your program doesn't need to know all the nitty gritty about dealing with the networking protocols.
There are a bunch of libraries and a big community of people using the ESP8266 in this format and others like the WiFi shield and SparkFun Thing. Getting it connected and talking to other devices on your LAN or the Internet is simple.
The ESP8266 supports APSD for VoIP applications and Bluetooth co-existance interfaces, it contains a self-calibrated RF allowing it to work under all operating conditions, and requires no external RF parts.
See the resources section below for how to get started with this module.
Note: The ESP8266 Module is a 3.3V device so please use an external Logic Level Converter if hooking up 5v sensors.