The widest of the Voigtlander for Nikon mount lenses, the 20mm Skopar is a manual focus lens but does have electronics to allow the camera to set autoexposure and white balance, and on D300 and above bodies, the camera will confirm focus for you.
This is an amazingly small but very sharp lens, reasonable competition to the Zeiss 18 and 21mm lenses from a sharpness standpoint, and reasonably priced too.