Printables has paid models now if you want to support a designer. I remember being recommended MyMiniFactory for more premium print a while ago.

With that said, I think it depends on what you are looking for and what you are trying to make. I see a lot of comments recommending Blender and FreeCAD. So I wanted to give you alternatives.

With that said, Blender is for 3D Modeling (think molding with Clay), where as FreeCAD is more like blueprints. Both have a high learning curve, and Blender has a large community to help new users. FreeCAD is getting there, but has a steep learning Curve. TinkerCAD and Fusion from Autodesk may get some hate here (for many good reason), but are the easiest softwares to get into if you want to make mechanical design.