Nikon and Canon make fantastic ultra-wide lenses for their APS-sized sensors sensors, and while I would not hesitate to recommend them, I would say that Nikon and Canon are so proud of them that only larger-than-individual entities like newspapers and photo studios can actually reasonably afford them. For the rest of us I offer this: the Tokina AT-X 124 PRO DX II.
I have used this well-built, optically excellent lens for my fine art/hiking work since the summer of 2005, and when it came time to replace my old 14mm at my newspaper, I had them buy me the same lens for day-to-day work.
At about half the cost of the mainstream equivalents, it is, in my opinion, just as sharp in all the situations that matter, and is built very nicely. After all, I use mine at work every day, and so far, no glitches. I highly recommend this somewhat hidden pearl.