PLX Kiwi WiFi is the best
The Kiwi WiFi from PLX Devices is the best on the market currently. It's the smallest and cheapest ($149) and has worked flawlessly for me. I've built mobile phone and desktop software around it, as it uses the ELM327 AT command set, making it simple to code for.
OBDKey (http://www.obdkey.com/wifi/index.asp?) also offers a WiFi dongle, but I found that the packaging was too long to be usable, as the case would stick out and hit my leg while driving.
Hope this helps.