Blogging.

EDIT:

It would be nice to look back at a decade of blogging, but I only started like ~3 years ago. It has many, many benefits: Practicing written communication, organizing one's thoughts, build up credibility, learn things in-depth by teaching etc.

Now that I think about it, actually, publishing anything in general. Working out in the open. I often kept side project to myself. I should have just open sourced that (did that for some projects after the fact).