Blender is a 3D modeling suite that can be used for modeling, UV unwrapping, texturing, rigging, water and smoke simulations, skinning, animating, rendering, particle and other simulations, non-linear editing, compositing, and creating interactive 3D applications, including video games, animated film, or visual effects.
In Zoo Tycoon 2 Modding, blender is used for modeling, weight painting, UV unwrapping and sometimes texture painting. As it is free, it is used by a large amount of modelers. In accompany of Python, PyFFI, and the blender nif scripts, it provides the best solution to create new models for the game. Exported models have to be fixed up using Nifskope.