A-Frame
A-Frame is an open-source web framework for building virtual reality experiences. It is maintained by developers from Supermedium and Google. A-Frame is an entity-component system framework for Three.js where developers can create 3D and WebVR scenes using HTML.
ApertusVR
ApertusVR is an embeddable, open-source, framework-independent, platform-independent, network-topology-independent, distributed, augmented reality/virtual reality/mixed reality engine. It is written in C++, with JavaScript and HTTP Rest API.
ARToolKit
ARToolKit is an open-source computer tracking library for the creation of strong augmented reality applications that overlay virtual imagery in the real world. Currently, it is maintained as an open-source project hosted on GitHub.
AR.js
A library to allow the development of marker-based, Natural Feature Tracking and location-based AR applications on the web. It can be used in conjunction with A-Frame (virtual reality framework) or three.js.