Modern power systems are increasingly evolving Cyber-Physical Systems (CPS) that feature close interaction between Information and Communication Technology (ICT), physical and electrical devices, and human factors. The interactivity and security of CPS are the essential building blocks for the reliability, stability and economic operation of power systems. This paper presents a web-based interactive multi-user power system simulation environment and open source toolset (W4IPS) whose main features are a publish/subscribe structure, a real-time data sharing capability, role-based multi-user visualizations, distributed multi-user interactive controls, an easy to use and deploy web interface, and flexible and extensible support for communication protocols. The paper demonstrates the use of W4IPS features as an ideal platform for contingency response training and cyber security analysis, with an emphasis on interactivity and expandability. In particular, we present the use cases and the results of W4IPS in power system operation education and security analysis.