What is logical upgrade for 18-105mm DSLR lens?

Currently, i have only 18-105mm kit lens on my Nikon D90

There's no logical upgrade… Though there are some questions to ask:

1) how long have you had the DSLR/lens?
2) have you done all you can with the camera and lens?
3) what do you want to shoot?
4) have you learned all you can about photography?

there are different lenses for different subjects… Wide angle, telephoto zoom, macro, prime…

I agree with the answer above.

It really all depends on what you are planning to shoot.

Macro = Lens stand offs are cheap and can add to your current lens for macro.
Convenience= I would move to an 18-200 or equivalent.
Sports & long range = up to 200mm with a 2x converter,
Weddings = same as for the convenience. And then accumulate the portrait lenses as well as the f2.8 fast lenses for the bag.
Portraits = A fast 50-70mm lens f1.8-f1.2
Landscapes?= Go wide

That fully depends on what that lens is not doing for you. Buddy of mine went to a sigma 24-70. I skipped the 18-105 and went straight for the 16-85.

Next upgrades are

1) A zoom lens with multipurpose wideangle plus long range… Generally 3 lenses fit into this category.
a) 18-55mm (b) 18-105mm (c) 18-200mm

Next good zoom option is a)55-300mm or (b)70-300mm

2) A good 'portrait lens'
a) 50mm (b) 85mm (c) 35mm (d) a similar range lens with a flexible focal length

3) A sexy multipurpose lens (a) 70-200 f/2.8 ~~~~ but its super costly…

Now depends on what you want a lens for you can go for the upgrade…

Even i own a D90… Personally i own a 18-105mm, 50mm f/1.4d & a 70-300mm lens… But when i go for trip i go with the kit lens… If i was given the option of taking lens separately when i bought y camera, i would have bought a 18-200mm lens instead of the 18-105mm lens…