I don't know about the Tokina 11-16 that Sassan mentions, but the Tokina 10-16 fisheye is available in Nikon mount and has a good reputation amongst Pentax users (it's a Pentax/Tokina design collaboration). I've had a brief play with the Pentax badged version in a shop but have never used one in anger. Can be had for about £400
http://www.srsmicrosystems.co.uk/945...Nikon-Fit.html
Fisheyes do give a very different view of the world, and it's not just a matter of focal length. My Sigma 10-20 will give a very different view at 10mm to a 10mm fish eye. I find the Sigma at 16mm gives a different view to my Pentax 16-45 when it's used at the same apparent focal length. Once you're talking about ultra wides, there seems to be more to it than the number on the barrel, very much a case of try before you buy.
Ultra wide angles give massive DoF at quite 'regular' apertures such as f/8. A lot of my street candid photography is done with the Sigma set to 10mm, f/8 and manual focus preset to about halfway between the 3m and infinity on the distance scale. I find I can get anything at just about any distance acceptably in focus at these settings.
If you don't want a true fish eye then you'll find plenty of people who like their Sigmas. I have no experience of the Nikon version. I do find it can be difficult not getting things straying into the frame with the Sigma, lord only knows how you work around these problems with a fish eye........
"What are those brown bits on the bottom of the picture?"
"They're my shoes."
"What are those furry bits at the top of the picture?"
"They're my eyebrows."