From:

From:

From:

From:

From: https://qcad.org/en/

The Open Source CAD System For Everyone

QCAD is a free, open source application for computer aided drafting (CAD) in two dimensions 
(2D). With QCAD you can create technical drawings such as plans for buildings, interiors, 
mechanical parts or schematics and diagrams. QCAD works on Windows, macOS and Linux. The 
source code of QCAD is released under the GPL version 3 (GPLv3), a popular Open Source license.

From: http://www.cadlogic.com/

Draft It V4 is a powerful, easy to use 2D CAD Software package that is completely free 
to use. No restrictions means you're free to create, save and print your work with our 
unlimited free version. Start today and download by clicking the button below.

From: http://www.ptc.com/products/creo-elements-direct/modeling-express/

drawing

From: https://blog.neweggbusiness.com/over-easy/5-free-tools-to-draw-a-network-diagram/

CADE is a primarily a CAD tool
yED is a Java-based resource that run in Windows and Linux.
MaSSHandra uses a real 3D environment to create network diagrams