For algorithms (or things that aren't snippets of code, for that matter), I keep a notebook. Writing in a notebook with a pen forces me to make the learned item

For algorithms (or things that aren't snippets of code, for that matter), I keep a notebook. Writing in a notebook with a pen forces me to make the learned item as simple as possible (e.g. by making a drawing), and the "aesthetics" of writing mean that I also feel compelled to keep my notebook tidy and interesting to look at, I feel it gives me a sense of personal accomplishment (I was never good at "arts" and making drawings e.g. of how a graph algorithm works is my way of creating "art" ;)) For things that involve code, e.g. an actual implementation or particular snippet, I use org-mode. It allows you to embed code as part of a regular document. I keep both thematic documents as well as a file per day for loose notes (the idea being that stuff that I collect will eventually be polished up and make it into thematic notes, but that never happens).