Open Cobalt is a free and open source software platform for constructing, accessing, and sharing virtual worlds both on local area networks or across the Internet, without any requirement for centralized servers. Open Cobalt uses the Squeak software environment, which is an open source Smalltalk system freely available for Windows, Mac and Unix. As is true of almost any Smalltalk application, Open Cobalt has identical functionality on any supported platform. As a Smalltalk system, it can usually be updated while the system is running without requiring a restart.