Also many production-grade compilers (GCC/G++, Clang, OpenJDK, V8, and almost every new language that's come out since the 90s) are open-source. You can go read the commit logs & source code to see how they work, if you're diligent and willing to slog through them. There are certainly tricks that professional compiler writers use that aren't covered in textbooks (the big ones center around error-reporting, incremental compilation, fancy GC algorithms, and certain optimizations), but you can always go consult the source to learn about them.

I thought the thread was really about domains where the bulk of knowledge is locked up in industry rather than being about rigor, but I'd put control systems in that category as well. Also information retrieval (Google's search algorithms are about 2 decades ahead of the academic state-of-the-art...the folks at Bing/A9/Facebook know them too, but you aren't going to find them on the web), robotics, and aerospace.