I bought the D7000 kit so I have the kit lens. I am wanted to expand, So I want to know
What do you all consider must have lenses? and why?
Like you I started with a D40 and a kit lens. Since I wanted to get the reach of a telephoto, I opted for the extremely versatile 18-200 VR. I used it for a walk-around lens for a long while until I started doing portraits. It was ok outdoors and in good lighting but immediately turned bad indoors so I ended up buying a 50mm/1.8 Series E lens which was a very cheap, yet good, manual focus lens. The next lenses I purchased was a 105/2.5 Ai-s, a 50mm/1.2 Ai-s. Then I got a 35mm/1.8 AF-s, a Sigma 10-20mm, a Nikon 80-200, then a 50/1.8D then finally a 17-55/2.8 which I am waiting to arrive.
That said. The very first lenses I purchased were Manual Focus because I was unsure of where my photography was heading. You should start knowing what your priorities are. If you have a kit lens, you have a cheap quality zoom that gives you the ability to shoot candids but if you take photos of indoor games you will be quick to realize it is not enough. You then have two choices, buy a cheaper fast prime lens or an expensive and heavy fast zoom lens. When you choose a prime, they are awesome fast lenses but you don't have the flexibility of the zoom. The fast zoom lenses are more expensive and heavier. And so on and so forth.
I suggest make the most out of your kit lens before you think about getting another lens. Know your photography, the limitations of the lenses and your priorities then start from there. The must-haves for a landscape photographer is different for someone who likes taking portraits. Someone once said, "The best lens you can ever have is the one on your camera".
My two cents worth.
*And oh, my favorite walk-around lenses is my 35mm/1.8G