Before TeX by Topic I was active in the Dutch TeX Users Group: NTG. This resulted in the "ntg styles" which these days are part of most TeX distributions.
Probably my most popular macros for LaTeX is a "comment.sty", which allows you to selectively include and exclude chunks of text. I use it for fancy stuff, such as writing exercises from my books to file, so that they can be reused for exams.
In the early 2000s I taught a course of topics in computer science, using TeX / LaTeX to motivate chapters on dynamic programming, NP-completeness, character encoding, and more.