GemPy

GemPy is an open-source, Python-based project for generating 3D structural geological models. Utilizing an implicit modeling approach, it enables the creation of complex combinations of stratigraphical and structural features, including folds, faults, and unconformities. Moreover, GemPy is specifically designed for probabilistic modeling to tackle parameter and model uncertainties effectively. 

Best of all: GemPy is completely free and open-source!