2D or 3D?

For 2D I used Draft Sight. Pretty much an Autocad knockoff. Will save files in several formats. Made by Dassault, the producers of SolidWorks.

http://www.3ds.com/products-services/draftsight-cad-software/free-download/

Never found a good 3D application. They all had some annoying quirk which made them incompatible with most accepted drawing programs.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now