I write down important stuff for later reference.
I used to do it in notebooks made of paper, but they are hard to edit,
hard to share, and quite nonportable.
So now I use digital formats, like PDF and HTML.
There is no single format that is appropriate for all of my notes.
As a result, my notes are stored in different places,
and each place has a different format.
Here I'll provide links to most of them.

 TheoremDep is a website that lists many theorems and shows the dependencies between them.
Theorem X is said to be dependent on theorem Y iff Y is used as a lemma in the proof of X.
 I created TheoremDep to better organize and document my own knowledge.
TheoremDep mainly contains theorems on abstract algebra, linear algebra,
number theory, graph theory, binpacking and probability.
 Theorems are authored in JSON+Markdown+MathJax.
ConcepDAG
converts them to an HTML+MathJax website.

 Some of my notes don't fit into the ‘graph of theorems’ representation,
so instead of adding them to TheoremDep, I use a more selfcontained format for them.
 The notes are authored in LaTeX and markdown.
LaTeX is compiled to PDF and markdown is compiled to HTML.

 A graphical representation of reductions among NPcomplete poblems.
 I extended this into a general tool for visualizing graphs
(source code),
but I haven't yet used that for anything.