FreeCAD is an parametric 3D modeler that lets you sketch geometry -constrained 2D shapes and use them as a base to build 3D objects. The program also allows you to easily modify your design by going back into your model history and changing its parameters.
FreeCAD reads and writes to many open file formats such as STEP, IGES, STL, SVG, DXF, OBJ, IFC, DAE.
Many user-contributed addons/macros are also available, for example to let you easily create screws and bolts, instantly construct lego bricks, etc.
