The majority of external keyboards available for Windows Mobile use either the Bluetooth or Infrared ports, with the handful of USB solutions totally unsuitable for any form of business or communications use. External keyboard devices usually ship with a stand or mount for the Windows Mobile device to be used as a mini monitor.
One of the main manufacturers of external keyboards for Windows Mobile devices are iGo with their Stowaway range. Other devices are available, however all of these external keyboard types share two common factors: price, and a lack of uniform technology. Purchasing a keyboard for a Windows Mobile device is completely different to popping to the local mall or supermarket and picking a new one for your desktop PC.
A popular choice is the infrared or bluetooth laser keyboards - devices that project a keyboard onto the desk - however these are considerably more expensive than the solid keyboard solutions.
Before choosing your device, check whether bluetooth of infrared would be more suitable for your device, budget and even battery life.