What wide angle lens should I buy?

With today's consumer level DSLRs you need to consider the cropping effect.

Smaller sensors give a cropping effect to the image, making field of view smaller for any given focal length. Simply, lenses give a more zoomed-in feel on crop sensors (DX) compared to full-frame (135 film, FX). It's good to note that given their popularity (especially in the past with film), full-frame focal-lengths (and equivalents) are often the standard focal lengths to compare to.

The difference is about 1.5x for Nikons DX to FX comparison. So, a 35mm lens, which is a bit shorter than a standard lens for full frame, would give you the same field-of-view as a 52.5mm on a full-frame camera, and that is considered to be a 'standard' lens.
Here's how I think of it (this probably varies among photographers) For full-frame:

• <28mm = ultra-wide
• ~28mm = wide-angle
• ~50mm = standard
• ~85mm = portrait
• >100mm = telephoto

Equivalent DX ranges (what you have):

• <18mm = ultra-wide
• ~18mm = wide-angle
• ~35mm = standard
• ~50mm = portrait
• >70mm = telephoto

My recommendation for a Nikon wide-angle? Grab the Nikon 18-55mm f/3.5-5.6G DX
It isn't very fast and it won't turn any heads, but it is a nice, sharp lens. Ken Rockwell praised its sharpness and said it was a bargain compared to other lenses in the same focal length range.  Similarly, Thom Hogan praised its value and optical performance, but criticized its handling, noting a diminutive focus ring that rotates during focusing, with no manual override. Anything wider (ultra-wide territory) is going to cost significantly more (>$500). I don't normally suggest zooms, but at ~$150 used, it's hard to beat. And the VR makes up for its small aperture if you're not shooting anything that moves.