WiFi infrastructure is everywhere which makes easier to make network of devices connected to WiFi.
There is also great range of WIFI routers with different space coverage and speeds, and as they are mass produced product the prices are at the rock bottom if we have to compare to Zigbee or Bluetooth.
WiFi stack require resources though and not quite good for smallish 8bit populary hobby processors, this is why UART-to-WIFI solutions were made and become popular.
Texas Instruments released CC3000 which cost about $20-25 in single quantities and $10 in volume.
It took no longer and Chinese company Espressif released their ESP8266 highly integrated UART-WIFI bridge IC, it requires just quartz crystal and balun to make WIFI module, their reference design is with just 11 x 12 mm board space!
And of course this comes at Chinese prices: you can buy ESP8266 module in single quantity from Seedstudio for $6.95 or from Alibaba for $5!
On top of this you can easy connect devices to Internet and send and receive data through UART!
This will definitely wipe out the $50-60 Arduino WIFI shields and even Microchip’s MRF24WB0MA etc modules.
We are going definitely to release MOD-ESP8266 which to connect to all our boards with UEXT and give possibility to connect to Internet with simple AT commands!
With MOD-IO2+MOD-ESP8266 for instance anyone could make WIFI enabled relay and Inputs and monitor through web page the inputs and drive relays for extremly low cost!