I have written a lot of style files (in plain TeX, I am actually a quite passive LaTeX user) for things like handouts and tests I use in my teaching, including my side job as a flight instructor. Some specialized typesetting tasks, like multicolumn dictionary typesetting (was another side project) took a lot of macro writing, as well. I have also written a number of parsers in TeX for pretty-printing code (I use another great Knuth's invention, literate programming, quite a bit). I am addicted to TeX, I can see it now :)