ResInsight is an open source project developing a cross-platform 3D visualization application for reservoir models and simulations. The main motivation of the development work is to improve key aspects of the human interaction when working with large scale reservoir models. The paper illustrates visualizations of properties, faults and wells designed for efficient interpretation of reservoir simulation data. To be highly responsive during visualization of large datasets, the system exploits multi-core CPUs and GPUs by a new core visualization library and a limited-intrusive multi-threading approach. Powerful and flexible mechanisms for result manipulation and numerical computations are established based on GNU Octave. Derived results can be returned to ResInsight for further handling and visualization. Eventually, derived and computed properties can be directly exported to form input to new simulations, thereby reducing the time needed for simulation cycles and parameter studies.