I personally have flip-flopped on this issue in the past. I have tried the "Email for permission" scheme, and after about a month of that I wanted people to stop emailing me, LOL. Now any freebies I do are free for commercial renders, because if someone can make a buck rendering an image with something of mine, more power to them. I made the item in the first place because it's fun for me, I really enjoy modeling and I like seeing people render my stuff. :) -Trav