Many authors attribute 3D printing technologies with the potential to revolutionize the production process of physical goods. However, insights on what users actually print in 3D remain in high demand. For the first time, 3D printing allows us to study how user innovators diffuse their creations on a large scale. To better understand this, we examined the world’s largest 3D printing platform called Thingiverse. We examined 12,616 individual objects, which accounted for 50 percent of the platform’s total downloads. We found that most objects belonged to the so-called category ‘3D Printing’ and that the growth rates of the ‘Household’, ‘Models’, and ‘Toys & Games’ categories outpaced all other main categories. We argue that user innovators in 3D printing do not only innovate for the reasons present in theory. Besides the user innovation that serves a niche market and the user innovation that responds to consumer demands which are not yet known to firms, the activities on Thingiverse suggest a third form of user innovation. We label this form of innovation ‘substitutional design’. They are mere substitutes for known and available solutions, but innovative in their creation and production processes. Lastly, we investigated the complexity of the designs available on Thingiverse, exposing that — contrary to theory — designs tend to become simpler as the platform grows.