The technological advances in the last decades led to radical changes and paradigm shifts in the modern society, replacing the former way of thinking, organizing and acting. More and more traditional services migrate towards internet and cyberspace, the number of those who use digital services growing exponentially in the last several years. In this context, education is one of the main concerns of the society: people must learn and adapt continuously to cope with the current economic and social environment. In this regard, traditional methods are no longer sufficient, so various other forms of education emerged in order to supplement the traditional ones: online education, skills-oriented education, collaborative learning, education at work etc. A rather new, but growing, form of education is the use of massively multiplayer online (MMO) 3D virtual worlds to provide an immersive and entertaining, more practical approach, based on simulation, rich user experience, interaction and collaboration. The technology behind these virtual reality systems is pretty complex, so creating your own application is, in most cases, not possible and will require a serious amount of time, money and human resources. A solution to this problem is to use frameworks dedicated to virtual world creation and management, such as Open Wonderland, Open Simulator or Virtual World Framework. These frameworks contain pre-built open-source solutions that provide working server architecture and also multiple features and services for creating and customizing virtual worlds. Open Simulator is currently one of the most used virtual world frameworks as it is based on Second Life and has strong back-end server architecture able to handle hundreds and even thousands of users. The purpose of this paper is to explore and explain how the Open Simulator framework works, summarize all the provided features, as well as to present several relevant examples that are using the framework to create rich virtual environments. We will also explain some problems encountered when we used the current Open Sim viewers (clients). [ABSTRACT FROM AUTHOR]