IFrIT is a powerful tool that can be used to visualize 3-dimensional data sets. IFrIT is written in C++ and is based on the state-of-the-artVisualization ToolKit (VTK) and, optionally, uses a GUI toolkit Qt. VTK is public domain and Qt is open source for all platforms (including Windows).
IFrIT has its origins (and hence name) in a specialized utility designed to visualize ionization fronts in cosmological numerical simulations. But IFrIT has outgrown its origins and now can visualize general data sets as well.