This is a simple list of some latex packages that I found very useful and took surprisingly long to hear about. Those are mostly well known, but it can be useful in case you never heard of them before.
Todonotes
Simple package that allows you to write notes inside a draft paper. It has many options, like different colours, inline/on margin and so on.
Cleaverref
Improvement on the \ref{} command, so that if you want to reference
a lemma, theorem, etc, it extracts the name of the environment to
provide the reference.
That is, instead of writing Lemma~\ref{mylemma} you just write \cref{mylemma}
Latexpand
I often find it useful to split articles in between multiple files. This is also very useful when using Git.
However, when submitting the paper to an journal or latex, I can be
annoying to have to deal with multiple files. This is where
latexpand comes in, it creates a single file “flat” of your article,
so that it is all contained in a single .tex file, ready to submit.
Mindmaps with tutorial
Tool to make “mindmap like diagrams” with tikz. It is very easy to
use. See the tutorial above.
Tikizit
A GUI (that just means a graphical application) to edit .tikz files in an easier way, more similar to inkscape and so on. It is very easy to use, you can either export your figures (as .png or .pdf file) or simply embed the .tikz file directly on latex. Again, the second option is not only useful for keep files small, but also works well with Git.
Other useful lists:
I recommend this list by Jonas Devlieghere.