I do want to add this: I think you should certainly give credit when possible, and even notify the creator if you can. I mean, if someone used a model I created, I'd be interested in seeing what they did, commercial or not. But as I pointed out in my example above, sometimes it may not be feasible to credit and notify every single contibutor when you're dealing with a lot of stuff, nor to build it all yourself. Hell, to create a graphic novel, as many people have (though I don't think anyone here has ever made any money off that...), with lots of real world locations...you're essentially talking about building EVERYTHING IN THE WORLD!!! Don't expect a sequel...