Online editors
Goo Create
Read the interview about Goo Create
This great 3D tool allows the creation of interactive 3D experiences based on HTLM5. Unlike other editors, Goo Create works with visual creation (or visual programming). In other words, it does not require coding: users can interact with objects, selecting them and placing them within the scene thanks to the different functions available in the editor (move, etc.), as well as assigning behaviours to each object.
However, the lengthy learning phase associated with this feature, which relies on a lot of experimentation, sometimes adds to the complexity of Goo Create. The tool’s biggest advantage is to allow the creation of WebGL experiences without having to systematically import 3D objects or having to integrate code.
http://goocreate.com/

PlayCanvas
Read the interview of PlayCanvas' team
Just like Goo Create, this tool allows you to easily edit a project. Using the platform does not require an expert knowledge of programming or 3D rendering.
What makes PlayCanvas shine is that it lets you edit one project from different workstations.
https://playcanvas.com/

Webglstudio.js
Read the interview of WebGLstudio's creator
Unlike Playcanvas and Goo Create, this platform is entirely open-source. This tool, however, requires a greater knowledge of programming and 3D rendering to be mastered. Element behaviour is assigned through code, though a graph-producing function lets the user bypass most of that tedious work.
http://webglstudio.org/

Three.js editor
This is an open-source online library that indexes numerous projects, and makes the creation of new WebGL projects easier. Three.js was the first of its kind to initiate a simplification of 3D programming, and almost every project is developed using it. Where coding a simple cube in WebGL would take hundreds of lines of code in Javascript, using Three.js considerably reduces that amount. However, the platform requires a certain experience in development and familiarity with 3D.
http://threejs.org

Clara.io
Clara.io is a free and collaborative 3D modelling website, which lets you create, animate and render elements in WebGL. The platform offers a lot of models, and is designed to facilitate rendering: great number of formats supported, compatible with any recent browser or operating system, etc. The core concept of Clara.io is to incite users to share their creations and work together.
https://clara.io/

Mixamo
Mixamo is an Adobe-owned platform used for 3D in general. It lets you create characters thanks to a very extensive and visual-only customization tool (no coding required). Characters animation is done in a similar fashion. Mixamo does not, however, allow users to create elements or scenes: it is dedicated exclusively to creating and animating 3D characters.
https://www.mixamo.com/

Online viewers
Sketchfab
Read the interview of sketchfab's creator
Sketchfab is, in a way, the YouTube of 3D. The platform lets you browse and share 3D projects, scenes and elements. Users can create a profile (which acts as a portfolio) and follow other profiles. This site is well-known in the 3D world: it is the place to be.
https://sketchfab.com

Minko
Minko is a BtoB viewer. It works just like a normal viewer (3D model visualisation, streaming, sharing, cloud service) but caters to companies and agencies with its tailored service. https://minko.io

Cl3ver
Cl3ver also caters to professionals, though it is designed specifically for architects. This tool lets you create 3D interactive presentations. The site, specialized in buildings and architecture, offers a powerful engine for a very precise rendering.
https://www.cl3ver.com/
