The Nikon NIKKOR 16mm f/2.8 is Nikon's full-frame (FX) fish eye lens. Essentially the bigger sibling to the Nikkor 10.5mm f/2.8 fisheye lens which suits the APS-C or DX sensor format in the Nikon Range.
The 16mm Fisheye is a fairly specialist lens due to the image distortion at the edges of the frame. That said where you wish to exaggerate the edges of an image (for artistic effect), or you wish to capture spherical panorama's this lens is a brilliant lens to achieve those effects.
In the case of spherical panoramas, this lens is well suited due to its wide field of view of approximately 180 degrees from corner to corner (on FX sensors). With this field of view, a spherical panorama can be captured by taking 6+2 images. The 6+2 setup is fairly typical, but involves 6 images being taken (one every 60 degrees), and then one image of the ground (NADIR) e.g. directly below the six images, and one into the sky (ZENITH) directly above the images. This total of 8 photos (6 horizontally and two vertically) can then be used to create a spherical panorama (e.g. 360 degree image) with the appropriate stitching software.
The lens itself is fairly small, weighing in at around 300g. The lens has no built-in autofocus motors and as such is unlikely to be suited for DX cameras (where the field of view would be significantly reduced anyway). This lens has been around in various iterations for some time now, and typically retails at around ~ $900 USD new, or a few hundred dollars less (e.g. ~ $600 USD) second-hand – depending on quality.
The build quality of this lense is very good, and the optics perform well. With the f/2.8 aperture it is well suited to Astrophotography or low light photography if you can live with the distortion around the edges.
I personally own this lense, and whilst I do use it sometimes (mainly for spherical panoramas), other functions I have started to use the Tamron 15-30mm f/2.8 for due to the lack of distortion (but a lesser FOV).