Today lens manufacturer Sigma announced two new fisheye lenses made specifically for cropped sensor digital cameras (such as the 1.5x cropped Nikons and 1.6x cropped Canons). The first lens is a 10mm fisheye, yielding an approximate field of 180 degrees across the diagonal (only ~170 degrees on the Canon). This lens focuses down to a staggering 1.8cm from the lens glass - so underwater photographers will need to be careful not get get lint or bugs in their dome ports - lest the lens refuses to focus past the dirt :-)
The second is a 4.5mm circular fisheye lens which yields a complete 180 degree circular field of view. These lenses will be available in Nikon, Canon, and Sigma mounts, however, pricing and availability haven’t been announced yet.