I certainly agree that it is a cornerstone to open source graphic arts. I would say the first 5 things on any computer I use is GIMP, LibreOffice, Firefox, VLC, and 7-zip.
Thank you for mentioning Converseen. I had normally just used bash and imagemagick for image conversion, but had not seen anything suitable for someone used to IrfanView.
I'd agree that there really isn't a direct replacement to AutoCAD. The other issue is that for quick and 'dirty' creation, things like TinkerCAD can help you put a concept model together quite fast without installing a program. But since both of them aren't open source, they don't need comment time.
I've used QCad community edition (forked to LibreCAD) to make a 1:1 pattern of something I was cutting out. I've also used Blender to make a model of parts of a standing desk. I then used LibreOffice to sort out all the pieces and place them so I could make the desk out of a single 1/2" sheet of 4'x8' plywood.
Salome-Meca is on my to-learn list, which I believe is open source and included in CAELinux.