123D is a suite of 3D modeling, design, and fabrication tools created by Autodesk. It is a free online app that allows users to create 3D models from photographs, drawings, or from scratch. With 123D, users can turn their ideas into 3D designs, create realistic renderings of their projects, and even create 3D-printed prototypes. It offers a variety of features for designing and fabricating 3D objects, such as 3D modeling tools, texture mapping, real-time 3D rendering, and 3D-printing services. 123D also allows users to share their creations with other users. 123D is a great tool for hobbyists, educational institutions, and professionals who want to design and fabricate 3D models.
Discontinued According to the official website k-3d.org: The latest version of the "source code" is from November 2015. And the latest binary executable version is even older. According to its Sourceforge page (https://sourceforge.net/projects/k3d/files/K-3D Win32 Binaries /), the latest version is from June 2010.
Easy to use, cloud-based: runs in the browser on PC and MAC, has a platform for open source sharing of 3D models.
Blender is much more complicated than 123d.