Have you ever wanted to add just one more little thing to an existing model? A name, a mash-up of two STL’s? There are plenty of free tools to do it and below are just a few good articles about those tools. First up an video from TinkerCad, then a recent All3DP article that will cover 6 popular modeling programs. Quick links to download them are below. Here is an additional Pinshape article with 10 Steps to STL File Modification: A Beginner’s Guide.

BONUS: learn how to chop up models for printing in sections when you want larger than build volume prints.

Tinkercad
Tinkercad is an easy, browser-based 3D design and modeling tool for all. Tinkercad allows users to imagine anything and then design it in minutes.

FreeCAD
FreeCAD is a parametric 3D modeler made primarily to design real-life objects of any size. Parametric modeling allows you to easily modify your design by going back into your model history and changing its parameters. FreeCAD is open-source and highly customizable, scriptable and extensible.

SketchUp
Hobbyists, kids, and backyard spaceship builders all agree that SketchUp Make is the easiest, most fun, entirely free 3D drawing tool in the world. We think you will, too.

Blender
Blender is the free and open source 3D creation suite. It supports the entirety of the 3D pipeline—modeling, rigging, animation, simulation, rendering, compositing and motion tracking, even video editing and game creation.

MeshMixer
Meshmixer is state-of-the-art software
for working with triangle meshes.

MeshLab

MeshLab is an open source, portable, and extensible system for the processing and editing of unstructured 3D triangular meshes.

(via Tinkercad 3D Design Blog)

BONUS: Learn how to segment larger prints using NetFabb Basic.

NetFabb Basic
netfabb Basic is not just a viewer, it provides mesh edit, repair and analysis
capabilities to everyone already being or aspiring to become part of this fantastic,
growing, creative, high-tech industry called Additive Manufacturing,
Rapid Prototyping or 3D Printing.