I just bought myself my first really good piece of glass for my D300, a 17-55mm f2.8. (See photo of Sadie in pets to see one of the first pics taken with it. Window light at f2.8)

But I would recommend a 35mm F1.8 prime lens or (50mm if you are using full frame.) They are small, fast, light and relatively cheap. You can just toss it in your bag and it's there handy if for some reason you need a fast little lens.