The CC3000 Wifi Shield combines usability, price and capability. It uses SPI for communication so you can push data as fast or as slow as you want. It supports 802.11b/g, open/WEP/WPA/WPA2 security, TKIP & AES. A built in TCP/IP stack with a "BSD socket" interface. TCP and UDP in both client and server mode, up to 4 concurrent sockets. It does not support "AP" mode, it can connect to an access point but it cannot be an access point. It has a proper interrupt system with IRQ pin so you can have asynchronous connections. I
In addition to the Wifi module, the shield features a microSD socket, reset button, an onboard 3.3V regulator, and a level shifter to allow 3 or 5V logic. The antenna layout is identical to TI's suggested layout and the same components, trace arrangement, and antenna so the board maintains are used to maintain FCC emitter compliance (you'll still need to perform FCC validation for a finished product, but the WiFi part is taken care of). The integrated antenna doesn't sacrifice range though, as it's roughly comparable to a smartphone.
Each shield is assembled and tested, but you will have to solder the headers for attaching to your Arduino. You can find the Arduino library under the How To: tab.