A friend of mine who's just picked up photography has an Nikon D7000 with 18-105mm kit lens. He's looking for a new lens and asked me if I had any suggestions. I've never used the Nikon system so I have no insight into their glass, but I know you talented folks do.
He's on a $600 budget but let's throw that out the window for now. What are your absolute, must have Nikon lenses?