siunitx is a very pleasant package for defining custom units with
LaTeX macros to ensure accurate, consistent typesetting of different
units. As the name might imply, it handles SI prefixes and the like
very nicely, and makes it easy to define arbitrary new units.
After struggling with different SVG molecule drawing programs I
discovered chemfig, which is an excellent package that provides an
interface over TikZ for drawing molecular structures. It's a little
touchy, and the default bond angle of 45 degrees looks off (this can
easily be changed) but it is way more pleasant than needing to go to
another program and re-export an SVG whenever you discover a small
error in a formula. Especially if that program is ChemDraw running in
a VM.
Just a nice package that hooks into Pygments to provide code syntax
highlighting. Kinda annoying that it requires a separate program, but
probably easier to maintain given what horrors I've seen looking at
the source of other LaTeX packages.