Sandisk [1,2] has one of the smallest WiFi Cards probably available. The card will allow SDIO enabled devices to connect to HotSpots (802.11b Access Points) for fast wireless Internet connectivity. There is an alternative product by Socket Communications [3].
[1] http://www.brighthand.com/article/SanDisk_SD_WiFi_Card_Reviewed
[2] http://www.sandisk.com/consumer/sdwifi.asp
[3] http://www.socketcom.com/product/WL6200-480.asp
