Croquet OS is a web-based operating system for creating three-dimensional apps with multi-user functionalities that run simultaneously on any device. Croquet can be used for communication, online gaming environments such as massively multiplayer online role-playing games (MMORPGs), 3D wikis, virtual learning and problem solving environments, privately maintained or interconnected multi-user virtual environments, and more advanced functions such as highly scalable collaborative data visualization, resource sharing, and synchronous computation among multiple users.