TaoTeJared talked me into the Tokina 12-24 F4 and I love it. Glad I got it.... The fisheye is the only thing wider than the 24 prime and I personally would never shoot with a fisheye.
Shot with my 12-24mm all day today. Dropped it twice on the frozen street in the snow. Still looks works like new.
I have a 15mm Sigma fisheye and keep it for those very few times a year when it is useful. Got such a good deal on it, I just keep it. Personally I don't like when software "straightens" fish-eyes out. You can always tell when one was and to me it just looks odd enough it takes away from an image. In my mind, either go with the look or use a different lens and move your feet.
Oh well it could be a 18mm/f4 even. I just want something in the spirit of the 35mm/f1,8. Cheap, small, tack sharp... my body doesn't have a focus motor, but I wouldn't mind focusing manually, as long as I have metering with a lens.
The closest you are going to get are the Tamron 14mm, Nikon 18mm, Tokina 17mm and I think there is a Sigma 14 or 15mm aspherical as well. Used they are under $400 maybe lower if you work eBay for a while. At that price you can get a 12-24 Tokina which is much more useful.
Cheap, small, tack sharp. Thanks for that, I needed a good laugh today.