Blender is well known for its 3D modelling capabilities and is often used for creating 3D models for games, animations etc.
This excellent piece of software has just about everything you need to create polygon models. What is great about editing models in Blender (and other 3D modelling packages) is the ability to go right into the detail of the model and edit individual vertices and the faces themselves.
You can also import existing .STL files into Blender and work from there allowing you to improve on the many .STL files that can be found on the web.
An easy to use tool to create 3D printable STL models from simple 2D image files.
If you have 3D model files that you wish to convert to STL files then this is the tool you need to convert your files ready for 3D printing.