My Nikon AF-S DX Zoom-Nikkor 18-55mm f/3,5-5,6G II standard lense on my Nikon D90 broke recently so in stead of getting a new one, I spent a bit more for a slightly better one. I bought a Sigma 18-50mm F/2.8-4.5 DC OS HSM.
Having used the lens for my weekend vacation I have to say that I'm very satisfied with it. The extra aperture it brings (3.5 to 2.8) really helps, and the quality of my photos seems to be of higher quality as well.