Well, this isn't a business story, but:
We recently built a home. I used Inkscape to design the top-view plans. I had a canvas set to a 1m scale and did everything life-size. Walls, doors, electrics, water were all drawn on their own layers.
With Inkscape, I could draw our furniture and lay it out; see which rooms had to be larger, which smaller. I used it to design the 'flow'--or how we would move from space to space.
After that, I actually printed out A4 pages and gave them to our builder who used them to do the job!
We only had to go to a professional draughtsman for municipal approval -- Inkscape saved us a lot of money because we were in control and could change things quickly.
Other than that, I use it all the time for any kind of design. I have no proprietary software any more.
\d South Africa