18mm is wide angle for a cropped sensor dslr like the D90. Any focal length 16mm and shorter is consider an ultra wide angle lens which is probably what you are looking for.
This is the list of ultra wide angle lenses you can choose from for a D90 user.
Nikon 10-24 f3.5-4.5
Nikon 12-24 f4
Sigma 8-16 f4.5-5.6
Sigma 10-20 f4-5.6
Sigma 10-20 f3.5
Tamron 10-24 f3.5-4.5
Tamron 11-18 f4.5-5.6
Tokina 11-16 f2.8
Tokina 12-24 f4
Each lens has its own strengths and weaknesses and even with a short description of shooting landscape, I probably can't really suggest which lens for you.
However, there are several things to consider when making your choice.
1. budget
2. Focal length - how wide you like to shoot. Even though the differences maybe may be just 2mm between 2 lenses at its widest, this can mean as much as 10 degrees more or less in the field of view
3. range of zoom - depend on how and what you gonna use it
4. distortion - think I can safely say that all the uwa lens can produce photos with quite large curvilinear distortion especially at the corners and the degree and type of distortion varies from each lenses.
5. image quality like flare, chromatic aberration, sharpness, etc.
6. thread size or thread availability - this will affect your choice of filters or no choice of filters