One concern with CAD is a fundamental aspect of their architecture: How are curves, arcs and circles handled? All- too-many programs take the expedient route of representing them as as polygons or polylines. Academic paper discussing this G codes for the specification of Pythagorean-hodograph tool paths.
A note on curve approximation: The trivial case with four cubic beziers (one on each quadrant) has very low maximum error, IIRC on the order of 0.1% of the circle radius.
A note on vector drawing: Vector drawing is like to drafting with rules, triangles, compasses and a French curve, while pixel editing is more akin to colouring in spaces on a grid. Vector drawing uses lines (a straight path between two "nodes" which are x,y coordinates, one is the beginning, the other the ending), regular geometric forms (defined by nodes which denote each point at which lines meet), arcs and circles (defined by the central point of rotation and the radius therefrom) and Bézier curves (a mathematical description of an arbitrary curve which has four "nodes", one on-the-path, two off-the-path and one on-the-path) to allow it to express pretty much any shape or form.
A game to learn about forming such curves: http://bezier.method.ac/#
A note on units: Most CAD programs store drawings in generic units, rather than a specific measure. It is important to know what unit was used when a drawing was created so as to be able to import it at the right size. Forum discussion.
It is also of course, straight-forward to work directly in G-Code, accomplishing Design and CAM in a single step using programs which provide integrated text editing such as NC Plot and NC Corrector (the latter is listed on the Previewing G-Code page. A tutorial is listed on the CAM page.
Note: A number of programs listed on the CAM page include design features for such specialized things as PCBs, Thread and Fret cutting and Gears.
|2D drawing (Opensource)||Inkscape||SVG drawing program which can either export to DXF or directly to G-code using plug-ins.|
|2D drawing (Opensource)||Doodle3D||Freehand sketching program for iPad and Mac OS X (source available for other platforms) to extrude and revolve into 3D.|
|2D CAD (Opensource)||LibreCAD||Fork of Qcad. Free and OpenSource CAD application, cross platform.|
|2D CAD (Opensource)||Simple2D CAD/CAM||Windows app to cut simple shapes (pockets, mostly) and simplified gears, includes a geometry mode that allows geometry defined by lines and arcs. It also generates geometry from any OpenType font.|
|2D drawing (Opensource)||Cenon (Mac OS X, NeXT/OPENstep, Linux)||OpenStep CAD/CAM program which has transitioned to opensource.|
|2D drawing (Opensource)||Xfig||G-code enabled Xfig.|
|2D CAD (Opensource)||Dedalo||2d CAD drawing software written in python3.|
|2D CAD (Opensource)||Zcad||Simple CAD program with dxf2000 file format support.|
|2D CAD (Freeware)||OpenS-CAD, a simple 2D CAD application||Windows .exe (87.5KB) and source (225KB).|
|2D CAD (Freeware)||DraftSight||Free CAD software for DWG files by Dassault Systèmes (Windows, Mac OS X, Fedora and Ubuntu).|
|2D CAD (Freeware)||nanoCAD||Free tool for creating, editing, validating, sharing, and managing CAD models.|
|2D CAD (Freeware)||Siemens Free 2D||Free CAD software by Siemens (Windows). textbook available as a PDF.|
|2D CAD:iPad (Freeware)||Sketch It Make It Apple App Store||2D CAD for the iPad.|
|3D modeling (Opensource)||K-3D||K-3D is free-as-in-freedom 3D modeling and animation software. It combines flexible plugins with a visualization pipeline architecture, making K-3D a versatile and powerful tool for artists. Available for Windows, Mac OS X and Linux.|
|3D modeling (Opensource)||BRL-CAD||BRL-CAD is a cross-platform open source solid modeling system that includes interactive geometry editing, high-performance ray-tracing for rendering and geometric analysis, image and signal-processing tools, a system performance analysis benchmark suite, libraries for robust geometric representation, with more than 20 years of active development.|
|3D modeling (Opensource)||Art of Illusion||Free, open source 3D modeling and rendering studio. Features include subdivision surface based modeling tools, skeleton based animation, and a graphical language for designing procedural textures and materials. Default design software for the RepRap project.|
|3D modeling (Opensource)||HeeksCAD||Together with HeeksCAM comprises HeeksCNC.|
|3D modeling (Opensource)||FreeCAD||FreeCAD is a general purpose feature-based, parametric 3D modeler for CAD, MCAD, CAx, CAE and PLM, aimed directly at mechanical engineering and product design but also fits a wider range of uses in engineering, such as architecture or other engineering specialties. It is 100% Open Source and extremely modular, allowing for very advanced extension and customization. See also the Python-query-scripting language CadQUERY.|
See also the graphical version Ulticreator, an interactive front-end for CoffeeSCAD.
|3D parametric modeling (Opensource)||ImplicitCAD||Similar to OpenSCAD (above), but written in Haskell and using an internal geometric representation, as opposed to the polygon mesh used by the former|
|3D parametric modeling (Opensource)||NaroCAD||3D parametric modeling CAD application for Windows, based on OpenCascade.|
|3D modeling (Opensource)||Blender||Free, OpenSource and crossplatform 3D program. Allows modelisation, shading, rendering, animation, interactive 3D, compositing. Not a CAD software though it can be used for any kind of modeling. See Blender CAM for a tool to export files for EMC. If importing from Illustrator, see this forum discussion for a note on how to handle units. Tutorials.|
|3D modeling (Opensource)||Wings3D||Free, OpenSource and crossplatform 3D modeler. Wings 3D is an advanced subdivision modeler that is both powerful and easy to use.|
|3D modeling (Opensource)||Shapesmith||Parametric, Open-Source 3D Modeling in your browser, can do basic primitives and addition/subtraction operations and export to STL.|
|3D modeling (Opensource)||SolveSpace||Parametric 3D CAD program for Windows. See also its 2D predecessor, SketchFlat.|
|3D modeling (Opensource)||POVray||"The Persistence of Vision Raytracer (POVray) is a high-quality, Free Software tool for creating stunning three-dimensional graphics".|
|2D drafting/3D modeling (Opensource)||lignumCAD||Tool for designing furniture.|
|2D drafting/3D modeling (Opensource)||Salome||Generic platform for Pre- and Post-Processing for numerical simulation. Can be used as standalone application for generation of CAD model, its preparation for numerical calculations and post-processing of the calculation results.
|3D modeling (Freeware)||Caligari Truespace||Commercial program released to freeware.|
|3D modeling (Freeware)||DesignSpark Mechanical||Freeware software for Windows by RS Components International. Free version of SpaceClaim 2012.|
|PCB (opensource)||KiCad||Electronic design automation (EDA) software suite for designing schematics of electronic circuits and printed circuit boards (PCB).|
|PCB (opensource)||gEDA||Full GPL'd suite and toolkit of Electronic Design Automation tools.|
|PCB (opensource)||QS Cad||Tcl/Tk EDA Package. Schematic capture through to PCB layout. Has Part editor as well as editor for Schematic and PCB Decals.|
|PCB (opensource)||Stripboard Designer 1.0||Open source (BSD) app written in freeware Visual Basic 2008 Express for Wintel machines. See also Stripes, a GPL Java version.|
|PCB (opensource)||DIY Layout Creator (DIYLC)||GPL Java software for creating stripboard and universal board designs.|
|3D modeling --- Surfboards||BoardCAD||Opensource CAD/CAM-system (written in Java) for surfboards.|
|3D modeling --- furniture||SketchChair||Opensource CAD/CAM-system for flat-pack furniture.|
|2D CAM --- Jigsaw puzzle piece cutter (Online - Silverlight)||CNC Puzzle Maker||Program for cutting out puzzle shapes w/ tabs.|
|3D CNC --- foam cutting for model airplanes||Jedicut CNC||Program for cutting out wings and fuselages.|
|2D CNC --- sails||Sailcut CAD||Sail design and plotting software.|
Additional programs which may merit investigation
- http://sourceforge.net/projects/technecadcam/ 
- GtkCAD --- (abandoned) modular, full featured, modern CAD/CAE/CAM system for GNU/Linux.
- JCad --- 2D CAD application written completely in Java. AutoCAD DXF native format.
- oCADis --- 2D CAD system for Linux. Basic drawing objects: line, arc, circle and more. Supports HPGL (export), DXF (import and export), PostScript output.
- ThanCAD --- 2dimensional cad, with raster inclusion capability and emphasis in engineering. Written in Python using the wxPython library.
- PythonCAD --- R38
- AnyCAD 1.4 Free Version --- Chinese CAD program available in English
- K3DSurf --- Program to visualize and manipulate Mathematical models in three, four, five and six dimensions. Supports Parametric equations and Isosurfaces.
- Frusta --- Tcl toolkit to manipulate data for a 3d-printer
- HYCAD 5.33 --- Free (abandoned? Last Updated: December 5th, 2007) CAD app for Windows from China by JiangJiang (developer homepage English link is 404, main site times out). Simplified AutoCAD interface, mouse gestures.
- LiteCAD --- 2D CAD program.
- A9 CAD
- Replath --- a program that can plot various 2D file formats either directly to a RepRap or to a GCode file.
- eCabinet --- CAD for custom cabinet makers (requires membership/registration)
- gCAD3D --- 3D - CADCAM - software for MS-Windows and Linux (available in German and English)
- MeshLab --- an open source, portable, and extensible system for the processing and editing of unstructured 3D triangular meshes.
- Misfit Model 3D --- an OpenGL-based 3D model editor. It supports multi-level undo, skeletal animations, texturing mapping, scripting, command-line batch processing, and a plugin system for adding new features.
- CADemia-Community --- free Java CAD software for true-to-scale technical drawings and documents. Also available in German
- Fandango: scripted 3d CAD --- formerly elcad, a python scripted 3d CAD application.
- AR-CAD - freeCAD and StCAD --- not the FreeCAD above, free 3D CAD Software with Motion Simulation Capabilities.
- The Piglet Drawing Editor --- 2-D engineering drawing editor that supports hierarchical object description, inspired by Hewlett-Packard's proprietary HP PIGLET (Personal Interactive Graphic Layout EdiTor).
- SagCAD --- Japanese CAD program
- AnyCAD --- .net Windows program / programming library
- GNU CNC CAM --- Tool for converting DXF (CAD) Files to G-Code (RS-274) NC-Files.
- Varkon --- parametric 2D/3D CAD, modeling and application development tool for engineering, design, architecture, etc. does 2D drafting, 2D and 3D modeling, sculptured surface modeling, and animations. Includes a 6-minute tutorial.
- CNCSimple ---3D modeling C# source code for Visual Studio Express 2010 at .
- Minos3D --- Windows program to avoid wireframe views.
- MonoWorks --- a full-featured, Open Source 3D Computer Aided Design (CAD) suite that includes object-oriented math and modeling libraries, as well as interactive viewing and modeling applications.
- AutoQ3D --- Opensource (GPL) version is AutoQ3D Community available for Windows here. Mac, iPhone/iPad, Android, Windows and Linux versions are available for AutoQ3D CAD.
- Anim8or --- Windows 3D modelling tool able to import and modify .3DS files
- netfabb Studio Basic --- Free application for Linux, Mac OS X or Windows for mesh edit, repair and analysis
- Wildcat CAD --- A modern 3D solid modeling kernel and associated applications for CAD and all types of related engineering. BSD license, last up-dated Oct 2008.
- CollabCAD --- 3D CAD/CAM Software system for collaborative design & development of Industrial Designs.
- pyturn a simple python code generator for cnc lathe programs
- 3DCrafter --- Real-time 3D modeling and animation tool that incorporates an intuitive drag-and-drop approach to 3D modeling. Free, Plus and Pro versions.