I recently upgraded my computer's tower case and wanted to use the old USB ports from the old case with the Arduino. There are 4 wires per port: +, -, D+, D-. How could I get the Arduino to act as a USB Host to drive a keyboard?
Take a look at an existing USB host shield. There is a link to a schematic at http://www.circuitsathome.com/products-page/arduino-shields/usb-host-shield-2-0-for-arduino.