Recently there have been several WiFi Arduino shields introduced to the market, however the cost of them can be close to the $100 mark. However if you already have an existing Arduino-Ethernet solution such as an Ethernet shield, EtherTen or EtherMega (etc) there is a much simpler and cheaper method of converting it to WiFi. Luca Dentella has documented how to use the inexpensive TP-Link TL-WR702N router as a WiFi client - and then by simply connecting it to the Ethernet socket of your hardware - you're done.
Furthermore, a quick Internet search finds these routers for less than $20. So to find out more, visit Luca's page here (Opens in Google Translate). And for more, we're on twitter and Google+, so follow us for news and product updates as well.
When putting together your next Internet-enabled Arduino project - save time, space and money with the Freetronics EtherTen. Apart from being fully Arduino Uno-compatible, it has onboard Ethernet, microSD socket, full USB interface (so you don't need a costly FTDI cable just to upload a sketch!) and supports optional Power-over-Ethernet.