Let me start by saying that I am fairly new to photography, but in a span of about 6 months, I have collected three great lenses, but the Nikkor 18-135mm f/3.5-5.6G is the one that is ALWAYS on my camera body. This is the perfect "walk-around" lens, and I would say I use this lens for about 80% of my photos.
I bought this lens when I purchased my D50 camera body. I could have also chosen a "kit" with my camera body, but after doing some research on the kit lens, I bought the body by itself, and added this lens to my purchase. Am I ever glad I did! I used my 18-135mm exclusively for about 4 months until I bought my next lens, and got some great photos with it.
The 18-135mm is a very functional lens- it can be used for landscape shots on the 18mm setting, or zoom with the 135mm and get some nice close-ups for portraits. Depth of field control is great here as well, although a tripod comes in handy if you get much about a f/11 depending on the lighting. I do have a little trouble if I am at the 135mm and shooting action photos, so now I just switch to my 70-300mm VR lens for those. Otherwise, my 18-135mm stays on, and is also what I store on my camera.
The price is a bit steep for this lens, but with the clarity of your photos, and the use you get out of the lens, it is well worth it. The lens is fairly light, (13.6 oz), and not very bulky, so it is easy to store in a camera bag for travel. I love this lens, and if it is only in your budget to buy one lens for now, I would highly suggest thinking about this one- it really is great!