I've a Tamron 90mm and can't fault it, other than the lack of image stabilisation which is by no means a deal-breaker. As gad-westy says, they go for around £200 used, which is a great price, I think one sold here recently for £190.
As for zooms, if you're after a "standard" zoom and want to stick with Nikon, the choice (IMVHO) comes down to 18-105VR, 16-85VR, and 17-55.
The 18-105 probably won't be much nicer than the kit lens (18-135 on the D80?), and I don't think the D80 will automatically correct lateral fringing from the 18-105 which was introduced with the D90, which will correct. Can be bought for around £190 new, I had one and didn't keep it long, as I bought...
A 16-85VR, which I like a great deal. I spend a lot of time at the wide-end, and the 2mm does make a difference to me. The quality of the lens is lovely in my opinion, but it is quite pricey at £400ish, down to around £300ish used. It's my walkaround, everyday lens, does almost everything I want. The only thing it could use is a faster aperture, which is where you need a...
17-55 f2.8. Lovely, high (pro) quality lens, basically the DX equivalent of the workhorse 24-70mm on full-frame, beloved of most FX users. £1100 new tho, and they seem to heamorrhage money, used ones can go for a little over half the new price.
If you're looking at Tamron/Sigma etc, they're a bit of a mine-field of models and performance, and there's apparently a great deal of variation between samples, especially among the f2.8 zooms. I recently looked at getting one of the 17-50 2.8 zooms but have given up on the idea unless one comes up dirty cheap used, and I can have a go with it before buying to ensure there's no performance issues.
HTH
