The Sigma 10-20 f4-5.6 is certainly worth looking at as well.
You said not not quite wide enough, so if you're simply looking for a bit more width, maybe a lens starting at 16mm might be a choice. That 2mm difference from 18mm may seem small, but at wide angles every mm is noticeable. Nikon do a 16-85mm.
I would also mention the possibility to build panoramas from multiple shots ( using software like open source Hugin. This allows you the flexibility of a normal zoom lens in a standard range while having the option to make some wider images from two or three overlapping shots. Not suitable for scenes with substantial movement, although you can sometimes manage to cope with a little movement. Worth considering for some purposes at least.