From the website:

Croquet is a powerful new open source software development environment for creating and deploying deeply collaborative multi-user online applications and metaverses on and across multiple operating systems and devices. Derived from Squeak, it features a peer-based network architecture that supports communication, collaboration, resource sharing, and synchronous computation between multiple users on multiple devices.

Every part of the system is designed around enabling real-time, identical interactions between groups of users. Croquet's architecture is designed to make it easy to develop deeply collaborative applications without having to spend a lot of effort and expertise in understanding how replicated applications work.

Using the Croquet Software Developer's Kit (SDK), developers can create and link powerful and highly collaborative cross-platform multi-user 2D and 3D applications and simulations - making possible the distributed deployment of very large scale, richly featured and interlinked virtual spaces.