Lego / Meccano style blocks for 'serious' projects
Here is what comes to mind (by no means exhaustive):

billed as "the Industrial erector set", 8020 is legitimately used in industry for that kind of purpose, and if I remember correctly, pretty expensive. It is the most "professional" thing I can think of.

OpenBeam styles itself as the the cheaper, "open sourced" 8020. I've frequently heard of it being used in 3d printers.

These are not as serious or professional, but might be worth mentioning:

MINDSi is pretty close to the mindstorms, but tries to be more rigid/durable. It's aimed at robotics.

Servo City's actobotics is an aluminum modular building system. It is also sold on sparkfun.

I have not used these enough to give good comparisons, but I hope one of these will be what you are looking for.