Many hire persons to rewrite certain articles and then to republish them in their own site, trying so to avoid duplicate content penalty. How legal is this? I am not very sure and I think all depends on the article author and I assume the more the article is changed, the more legal it gets.
However, if you cannot afford or you dont want to hire human rewriters, you can use this free tool.
I've seen other tools too, but most of them were commercial or hard to use so I could not really test them.