Sigma 4.5mm f/2.8 EX DC HSM circular fisheye Currently, Sigma offers a couple of interesting extreme wide angle lenses-4.5mm f/2.8 EX DC HSM circular fisheye lens and the AF 8-16mm f/4.5-5.6 DC HSM lens, both of which are readily available to photographers right now.
The former offers a close-up focusing distance of 5.3 inches and, like Nikon's 6mm lens, is used for scientific applications as well as more creative photography. It's a lot smaller, lighter and less expensive too; the MSRP is $1,500 but you can probably pick it up for less.
Sigma's 8-16mm lens (designed for APS-C sensors, with an effective focal range of about 12-24mm), is a rectilinear wide angle lens with a twist. Our resident lens guru, Julia Silber, is currently testing Sigma's 8-16mm lens for PopPhoto, and reports that you can see the outline of the rectangle in the lens but you can add a special ring to the front of the lens to get a circular (vignetting) effect. (Actually, the ring is included so you can attach a cap over the lens' "bulb.") Like its sibling, the 8-16mm is tiny and inexpensive compared to the Nikon 6mm lens and has an MSRP of $1,100.