The popular tool now has a free version with 20 core functions.
Tripolygon has recently released UModeler Lite, a free version of the popular UModele toolkit. It has an intuitive sketch-based interface and convenient functions for you to design levels and work on new models.
UModeler Lite is said to contain 20 core functions of the UModeler full version that can be used for Level Design, 3D Modeling, VFX, and Concept Design, and more within Unity.