Blender

A complete 3D package best known from the animation world, but for an engineer it is an unusually strong visualisation and rendering tool: import models from CAD, apply realistic materials and lighting, and render at presentation quality with no licence cost. It also carries physical simulations of rigid bodies, cloth and fluids — enough for illustration and behaviour checks, not as a substitute for engineering analysis — and a Python scripting engine for building custom tools. The learning curve is steep, but justified if you produce visualisations regularly.