Malcolm Jones in The Daily Beast: There’s Nothing Wrong—and a Lot That’s Right—About Copying Other Artists:

"Copying, like rote memorization, is no longer in fashion. For centuries, student artists copied plaster casts and worked up variations on images of the Holy Family. The job then was as much craft as art. Then came Romanticism and the cult of self, which needed to be expressed. Then Modernism blew the doors open with its insistence on constant change that now permeates—and rules—every corner of the creative world (Ezra Pound’s dictum, “Make it new!” might as well have an “Or else!” tacked on). And that’s fine if you are a true artist. Alas, most of us aren’t, so when our puny efforts at creativity fall short, we feel like failures and quit before we’re out of grade school. Ever thereafter, we regard art as some mysterious, gated territory where we cannot go. Somehow I don’t think that’s what our teachers intended."

I love copying other people's work. You learn about the process, you notice details that you wouldn't have otherwise, and it's fun. Eventually, your own work gets better as well. I've done this for years with guitar solos, with computer programs, and more recently (well, over the past couple of years) with sketching.

There's nothing wrong with this— just keep it to yourself. And if you do share, don't claim it as your own. It's as simple as that.

When I was working on the bezier tools for Acorn 4, I would keep an eye out for awesome vector art and try to re-make it in Acorn. At times I found Acorn's tools lacking so I would tweak it or add new features. And that's the primary reason Acorn's vector tools got a big jump in version 4.