Do Tamron and Sigma lenses fit on a D80? Which one is better? I have always just saved up my money and gotten a Nikon lense but I really want some new lenses...
yes, but make sure you get a nikon mount one. Best is subjective. They're both good brands, but I prefer sigma. I just ordered a 24-70 f 2.8 Sigma and have a 50-500mm Sigma, so I'm a little biased. But I've used a few Tamrons with no complaints.
Asking which is better could be equated to asking if Canon or Nikon is better. It'd be more accurate to find specific lenses that you're interested in from either manufacturer and get opinions about the specific lenses. They make lenses for all sorts of different brands and mounts, so you just have to be sure that you buy the appropriate Nikon mount.