Versatile lens for Nikon D90?

I have a Nikon D90 with the kit 18-105 and a 35/f1.8. I'm looking to get another lens preferably a f2.8 zoom (I don't need extended zoom) that works well in low light. I would be using it for everyday photography and light portrait work (mostly candid). I was thinking about the Sigma 17-70 f2.8. Any recommendations/comments?

The Tamron 17-50/2.8 VC has a very good reputation. It's obviously not as long as the Sigma though, and thus less suitable for candids. There's also a Tamron 28-75/2.8, which may be just what you're looking for. It seems to have autofocus issues in low light though, according to this review:
http://www.the-digital-picture.com/reviews/tamron-28-75mm-f-2.8-xr-di-lens-review.aspx

If you're comfortable with a 50mm wide end and no image stabilization, give the Tokina 50-135/2.8 a thought.

Most zooms will have AF issues in low light.

The sigma 17-70 is only 2.8 at the wide end. Apart from that, it does just about nothing your 18-105 does not do.

As said, tokina's 50-135 is one very interesting option for candid portraits. Another rare but not that expensive option is the old tokina f2.6-2.8 28-75mm.