I believe that it might be a good idea to keep a personal mathematical diary, recording your readings, questions, thoughts, research ideas, etc. Surely one can choose to keep paper-based journals, but electronic ones are easier to store and manage.
I am aware that there are numerous software for journal keeping, some of which are free of charge (and good!). But I also noticed that (almost?) all of them do not work well with mathematics. It may not be easy to write a simple formula such as $A=\pi r^2$, let alone a commutative diagram, or more complicated stuffs.
So I wonder if anyone knows any diary-keeping software (free ones preferred) that works well with mathematics; namely, it is very easy to input mathematical expressions, as one would do with $\LaTeX$.
Tim Gowers recommended TiddlySpace. It has a MathJax plugin.
I tried it and found it easy to get started, but then I didn't keep using it. But it seems to me like it would be good for mathematical journaling.