I've been using Microsoft Visio for my drawings for many years. Most people think of it as a business graphics program, but it's far more capable than that. I do all of my design work with it, here are a few examples:
When I was building my LJ-based "Retro Wrangler" pickup, I did all the dimensioning and frame modification design with Visio:
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And here's something much simpler, this is part of an ammo can/battery tray design that goes inside over the rear inner fenders of an LJ, I carry and ammo can on in one on the passenger side and my auxiliary/kitchen battery in one on the driver's side:
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I do lots of concept images using a combination of Visio and Paint Shop Pro, this image was one of the first ones I did when designing the Safari Cab hardtop:
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Funny thing about the image above, back when I did it I was involved in some things with Microsoft and I shared the image with them. They couldn't believe it was done in Visio and asked me to serve on their Visio Customer Advisory Council, which I did for a couple of years.
A few other concepts - this TJ photo was sent to me by someone who was considering buying a TJ Safari Cab from Gr8Tops, in the original image the TJ had a soft top. I replaced the soft top with a Visio-drawn Safari Cab...
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In Visio you can draw shapes using lines and other basic elements like all other drawing programs, but a nice feature of the program is the ability to create palettes of shapes that you can drag and drop onto your drawing. In this screen shot, a palette of Jeep and Safari Cab parts is at left, those shapes can be dragged/dropped onto the drawing to quickly create a drawing of any TJ/YJ/CJ with any Safari Cab configuration. In this image I'm in the process of dragging a half door from the palette to the drawing (the mouse pointer didn't show up in the screen shot but it's over the half door):
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And finally, I've done dozens of illustrations for magazines and since I generally get paid for those illustrations I guess that makes me a semi-professional illustrator. Here's an example, this drawing of locomotives was for a railroad magazine; they're narrow gauge locomotives on the Welsh Highland Railway in Wales:
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I suppose one of these days I'll give up Visio for a 3d CAD program, but Visio is so capable I hesitate to give it up.