MatterControl is a free app that lets you organize and manage your 3D prints. It's designed to help you get the most out of your 3D printer - making it easy to track, preview, and print your 3D parts. MatterControl allows you to create a library of parts and queue multiple files for printing. Slicing is integrated and done on an as-needed basis. Ongoing development of MatterControl is supported by MatterHackers.
SketchUp is a 3D modelling program for a broad range of applications such as architectural, civil, mechanical, film as well as video game design — and available in free as well as 'professional' versions.
SolidWorks is a 3D mechanical CAD (computer-aided design) program that runs on Microsoft Windows.
OpenSCAD is a software for creating solid 3D CAD models. It is free software and available for Linux/UNIX, Windows and Mac OS X. Unlike most free software for creating 3D models (such as Blender) it does not focus on the artistic aspects of 3D modelling but instead on the CAD aspects. Thus it might be the application you are looking for when you are planning to create 3D models of machine parts but pretty sure is not what you are looking for when you are more interested in creating computer-animated movies.
Blender is a free and open-source 3D computer graphics software product used for creating animated films, visual effects, interactive 3D applications or video games. Blender's features include 3D modeling, UV unwrapping, texturing, rigging and skinning, fluid and smoke simulation, particle simulation, soft body simulation, animating, match moving, camera tracking, rendering, video editing and compositing.