Drone simulation software is recently more and more popular.
So far we distinguish two main purposes for drone simulation:

• Research and development;
• Operator's training;

At the moment, there are multiple solutions, but two of the simulators are freely available thus they are popular among a variety of users:

• AirSim by Microsoft (still Open=Source): https://github.com/microsoft/AirSim; This simulator is also suitable for UGV, not only for UAV, and its purpose is mostly to implement and test high level, AI-related solutions for autonomous vehicles. AirSim can integrate with ROS-based hardware (Robot Operating System) solutions. As it is implemented using popular gaming Unreal Engine, the graphics is astounding yet it requires a powerful GPU to run. The simulator provides rich user experience with a natural-looking environment like cities, parks and so on.
• GAZEBO was developed as pure Open-Source simulator for a variety of physical devices, also for UAVs and UGVs, but not limited to: http://gazebosim.org/; Its origin is to simulate robots. It provides simple graphics, but the benefit is it can run on constrained hardware. The GAZEBO seamlessly integrates with the ROS environment. The simulator also provides ORCE rendering engine for a realistic environment rendering but the real power is simulator's flexibility on configuration and simulation running, starting from constrained Linux-based machines, finishing on Amazon AWS, cloud-based simulations.