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FLOSS CAD software:

https://brlcad.org/
BRL-CAD is a constructive solid geometry (CSG) solid modeling computer-aided design (CAD) system

https://sourceforge.net/projects/librecad/
LibreCAD is a Free Open Source 2D CAD application for Windows, Apple and Linux

https://www.openscad.org/
OpenSCAD is the Programmers Solid 3D CAD Modeller
http://www.openscad.org/downloads.html
http://scorchworks.com/ScorchCAD/scorchcad.html
formats: DXF, STL, OFF

http://solvespace.com/download.pl
SOLVESPACE is a parametric 2D/3D CAD
formats: DXF, EPS, PDF, SVG, HPGL, STEP, STL, OBJ,

https://www.freecadweb.org/
FreeCAD is an Open Source parametric 3D CAD modeler

https://archimedescad.github.io/Archimedes/
Archimedes is Free and Open Source CAD Software

https://chocolatey.org/packages/jwcad
http://www.jwcad.net/
jw_cad is a popular 2D CAD from Japan.

https://github.com/mkeeter/antimony
Antimony is a CAD tool based on LISP.

https://github.com/colah/ImplicitCAD
ImplicitCAD is a CAD program written in Haskell which handles. CSG, bevels, and shells, 2D and 3D and gcode generation.

https://github.com/qcad/qcad
QCAD Community Edition is a 2D CAD for Windows, Linux and Mac.

https://github.com/zecruel/CadZinho
CadZinho is a minimalist CAD program using the Nuklear GUI.


FLOSS 3D Printing Related Software:

http://www.salome-platform.org/downloads/current-version
SALOME is an Open Source Integration Platform for Numerical Analysis

http://www.k-3d.org/
K-3d is 3D Modeling and Animation software.

http://shapesmith.net/
ShapeSmith offers parametric, Open source, 3D modelling in a browser

http://slic3r.org/
Slic3r is an Open Source 3D printing toolbox.
G-code generator for 3D printers. Exports SVG slices.

http://dkobozev.github.io/tatlin/
Tatlin is a multi-platform Gcode and STL viewer for 3D printing

https://sourceforge.net/projects/meshlab/files/meshlab/MeshLab%20v1.3.3/
MeshLab is a mesh processing system for user assisted editing, cleaning, filtering and rendering of large unstructured 3D triangular meshes.

https://www.kicad-pcb.org/
KiCad is an Electronic Design Automation Suite

http://www.wings3d.com/
Wings 3D is an advanced subdivision modeler that is both powerful and easy to use.

https://wdune.ourproject.org/
White Dune is a VRML/X3D editor

http://www.gnuplot.info/
gnuplot is a 2D/3D plotter

http://www.artofillusion.org
Art of Illusion is a free, open source 3D modelling and rendering studio.

http://www.blender.org
Blender is a Free Open Source 3D creation suite.

https://www.libreoffice.org/download
LibreOffice Draw is a vector graphics editor.

http://bricksmith.sourceforge.net/
Bricksmith is a 3D virtual LEGO modeller.

https://github.com/OctoPrint/OctoPrint
OctoPrint provides a web interface for controlling 3D printers.

https://github.com/WEREMSOFT/stl-loader-for-raylib
stl-loader-for-raylib provides functionality to load, manipulate and render STL files on Raylib.


SVG software:

https://github.com/SVG-Edit/svgedit
svg-edit is a powerful SVG editor in a browser

https://github.com/cast-org/cwm-drawtool
CWM Draw Tool is an easy to use drawing tool based on svg-edit

https://wiki.gnome.org/Apps/Dia
Dia is a diagram creation program
formats: EPS, SVG, XFIG, WMF, PNG

https://inkscape.org/
Inkscape is a professional vector graphics editor


2D tools:

https://sourceforge.net/projects/ktoon.berlios/
KToon is a 2D animation toolkit

https://www.synfig.org/
Synfig is an Open Source 2D animation studio

http://www.pencil2d.org/
Pencil2D is a tool to make 2D hand-drawn animations

https://github.com/hlorus/CAD_Sketcher
CAD Sketcher is a constraint-based sketcher addon for Blender that allows creation of precise 2D shapes by defining a set of geometric constraints such as tangent, distance, angle, equal and more.


Other resources:

For more graphics related programs, see http://www.distasis.com/cpp/pbtc.htm#pb3