MissNancy is also correct, in that some (actually very few) people allow you to modify things and distribute them. One case that I can think of at the top of my head, is the generic Quick suits/dresses/shirts/coats at:

http://www.morphography.uk.vu/

Those can be altered to your liking and used however you wish. That was the whole intention of those models though, to give people a clothing starting point. Most other models, however, you can't do such a thing.

BUT, say you create something cool that goes with someone else's model. You CAN save that as a smart-prop for it and distribute the hell out of it. You made it, even if it is for someone else's original model. Just don't try and distribute the original model with your addition, remember to just post a link to what it goes with.

Edit: That 10% myth is actually true, just not for 3D modeling. It works for recipes, and some other products you find in stores. I know of a company that orders a certain popular brand of picante sauce, they add a few of their ingredients, put it in their jars with their labels, and sell it commercially. So yes, the 10% rule does apply somewhere, just not in the 3D world.

                                                                                                                    

Poser 10

Octane Render

Wings 3D