There is a relatively new product by Texas Instruments – the SimpleLink CC3000 WiFi module that is quite small and cheap – $12. Looks quite promising, size is about as much as a fingernail:
There is a KickStarter project using that chip already, quite intriguing. There is still no library for Arduino, but I guess it is just matter of time before one appears. There is an initiative here.
Other projects (in progress) that use that chip are the WiFiduino
..and a CC3000 shield project:
I will keep an eye on these projects, WiFi is something I long wanted to include in my projects.
Hackaday has listed an Arduino library for this chip http://hackaday.com/2013/06/24/tis-cc3000-wifi-chip-gets-a-library/
I have been following the topic; It is still very basic, just a test of the SPI communication, but lays the basis for further work.