Poly by Google is a library of free 3D objects for augmented and virtual reality apps.
threed.io is a browser based tool to generate custom device mockups on the fly. Upload your Designs, make adjustments and download your custom mockup! Even works pretty nice on mobile. We're still in beta, but if you're interested in threed, please read the about text on the site.
With the new Poly API, you can access our growing collection of creative commons 3D assets and interact directly with Poly to search, download, and import objects dynamically across desktop, mobile, virtual reality, and augmented reality.