Blender - Blender is the open source, cross platform suite of tools for 3D creation.
OctoPrint - OctoPrint is the snappy web interface for your 3D printer that allows you to control and monitor all aspects of your printer and print jobs, right from your browser.
FreeCAD - An open-source parametric 3D modeler
MatterControl - MatterControl is a free, open source app that lets you organize and manage your 3D prints.
Sculptris - Sculptris: Enter a world of digital art without barriers.
AstroPrint - Slice, Store, and Print your 3D files from any web enabled device.