Tamron are one of the oldest established independent lens makers around, and although many of Tamron’s recent lenses are at the budget end of the scale they have always made excellent optics, and even some of the cheaper lenses, like the 70-300, perform better than you’d think. In fact there are rumours that they make lenses for some of the camera manufacturers own brands. Some of Tamron’s more expensive DSLR lenses are amongst the best available. In particular, Tamron’s 17-50mm f2.8 is one of the best modern standard zooms around, and the Tamron 18-250mm is by far the best superzoom I’ve ever tested. Tamron lenses are available to fit Nikon, Canon, Sony / Minolta and Pentax / Samsung. (not all lenses in all the fittings though)
Note: If you have a newish Nikon DSLR check for compatibilty before buying even new lenses - if the lens doesn’t have an AF motor built in it may only work in manual focus.