I am a mathematican. Since math is written in TeX, I have come to enjoy tinkering with the language; at the moment, I have one package on CTAN, ytableau, for drawing Young tableaux and Young diagrams in a variety of ways, some of which are not possible with the other such packages.
I am a big fan of PGF/TikZ, particularly the
pgfkeys sub-package. Read my newer answer to onlyitems? How to select specific items from an item list to find out what I think is so great about it and how you can use it to avoid doing almost any programming task in basic TeX.