Anything you downloaded you can use. That's why the files are there (or were there.) One thing to be careful of - some models were posted that said there were not to be redistributed in any way. If any of your models/textures, etc. have that written into their readme files, then do not upload them to your site for others to download, and do not e-mail them to others. Instead, tell them how to contact the artist so they can get a copy of the model. It's an extra step or two, but considering the fact that you got the items for free, it's a small price to pay.