Zoom lense for D90

It all depends on how much money you are will to part with....  I shot my D90 with a Tamron 70-300 for a year or so.  I took some very nice shots with it.  However, like you i wanted a little more reach.  I went to a Sigma 150-500.  I had more reach, but less versatility.  I then bought the Sigma 50-500.

Most of my shots are outdoors, so far I have been pleased with the lens.  sigma has done pretty well, squeezing as much sharpness as they can, out of a lens with this much focal length.

Or you can spend thousands buying a prime 5 or 600mm.