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Founded in the Netherlands, Shapeways is currently based in New York. It's one of the most universally recognized 3D printing services provider, allowing clients to send in their own models to be printed and shipped, but also providing a marketplace for designers to sell 3D printed objects. This marketplace is known for the wide variety of objects offered, some of them high-end design creations.
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Pros
Pro Great selection of materials
Shapeways offers over 30 kinds of materials to choose from including platinum, sandstone and castable wax. The full list of materials can be found here. On top of the materials themselves, there are several finishes to choose from. Designers and clients can order sample material kits to make sure they get what they are expecting before requesting the print.
Pro Marketplace with an immense variety of objects to choose
Shapeways' marketplace is divided in many different sections: gadgets, puzzles, toys, games, Christmas, Hanukkah, jewelry... There really is a lot to explore and it's one of the most varied markets of it's kind. It's easy to lose hours and hours just browsing the shop and discovering things you never knew could be 3D printed.
Pro Industrial grade, quality printing
Shapeways uses industrial level technology. Their machines are capable of much more than most quality domestic 3D printers, therefore the final results are much better. On top of that, contrarily to what happens with other services, the files clients send in aren't automatically printed: they are reviewed by technicians to make sure the final result is as intended. There have been claims of 3D designs that could not be printed properly by any service, other than Shapeways.