Vehicular Ad-hoc Network (VANET) simulators have been gaining widespread attention in recent years and are crucial for conducting vehicular research. The fact that VANET simulators can simulate near real life scenarios to evaluate driver-safety applications without endangering human lives vastly contributes to their popularity. In this paper, we present the VEhicular NeTwork Open Simulator (VENTOS), a closed-loop VANET simulator that combines the capabilities of both communication network and vehicular traffic simulators. More specifically, VENTOS comprises of Simulation of Urban Mobility (SUMO) and Objective Modular Network Testbed in C++ (OMNET++). As an alternative to current proprietary Vehicular Ad-hoc Network (VANET) simulators, VENTOS is a free, open-source simulator that is designed for vehicular traffic flow analysis and can incorporate new control logic, such as intelligent traffic controller, collaborative driving, dynamic routing, and self-driving capability. VENTOS also allows for V2X (Vehicle-to-Vehicle or Vehicle-to-Infrastructure) communication through dedicated short-range communication (DSRC) integration. We discuss about the VENTOS architecture as well as various example applications that use VENTOS.