Good camera lens for travel photography?

I have a Nikon D5000 which I have had for close to 4 years now, so I know the camera very well.
I travel often and my standard 18-55 lens is beginning to bore me. I recently purchased an AF-S Nikkor 50mm 1:8 G to use for portrait photography and it delivers really well for the price.

I'm looking to add a perfect travel lens particularly focusing on landscape photography so any wide angle lens that you may recommend.
My budget would be a max of $700 with leeway to about $800 if the lens is significantly good.

For travel, I'd bring along your 50mm 1.8, and add a Nikon 18-200mm AF-S VR. I used the 18-200 as my only lens during a couple of weeks in China and found it replaced the 18-55mm and the 55-200mm. Having the VR feature was really handy when shooting "on the go". However, there were times in museums or temples where low-light was a challenge. At that time I didn't have an f/1.8 lens and now wish I could have brought one.

Just remember to bring extra batteries and formatted memory cards.

There really is no one do it all lens. If you want a good zoom, you want a constant aperture of f/2.8. Nikon makes excellent lenses, but you can buy nice Tokinas, Tamrons and Sigmas that are cheaper and almost as good and the Nikons. The f/2.8 is the 'pro' line of these lenses.

I have 2 zooms that I will use as 'walk around' lenses when traveling, am 16-35mm and and 28-80mm lens. For scenery, the first one is better, for portraits, the second is better. I rarely need anything longer than 80mm for travel. I have a 70-200 for sports, plus a couple of macro lenses, but I don't need these when I'm out.

If you want to focus on the landscape aspect, I would look at Sigma, Tamron or Tokins lenses in the 16-35mm range with constant f/2.8 aperture.

I used the same camera (D5000) till last year and had been doing a lot of travel photography.

I had started with Nikon 18-55. Then added to the bag a Nikon 55-200. Yet later I supplemented the bag by a Nikon 35mm f1.8. Then I moved on to Sigma 10-20.

Later I realized that its such a pain to keep changing the lenses while on the move. Also matter a lot is the weight I had to carry considering the fact that travel photography does include a lot of travel too!

Finally, I came across Tamron 18-270mm PZD lens. Covers a BIG range. From 18mm at the widest to 270mm at the telephoto. D5000 being a Dx camera, 270mm actually transforms to 405mm. I enjoy carring a single lens. No hassle to chage. Not to miss any photo.zoomin and zoomout at ease. Only one set of filters is enough for me

D5000 + Tamron 18-270mm + Nikon SB600 is my Travel combo and am extremely satisfied with it.

I also work on D800 platform. However, I do not use that for travel.

Tamron 18-270mm is about 500USD