Text files and grep to look back. Then when a project starts to have some longevity I skim through the notes and make more formal documentation of some aspects of the work I'm doing.
I think I'll always stay with simple text files for projects that may or may not go anywhere.
I recorded a brief screencast of how I keep a dev journal. :) https://youtu.be/4W8i8FTz_ck
Hope it's helpful!
After a lot of fiddling with different options I use Vimwiki running in separate Vim instance on dedicated virtual screen.
Hit ^6, hit enter, hit F4 to put the timestamp, write. Done.
I would recommend emacs org-mode. I've been using it for months now and became way more productive than before. You may want to take a look at this fine set-up. http://doc.norang.ca/org-mode.html
Devarist is built for keeping a work journal. It supports Markdown and images too.
Check it out, feedback is very welcome :-)
Your git commit logs... Because what you actually accomplished is all that really matters.
github seems like good option.
Evernote?
Plain text files are a great way to go. You should check out JRNL, https://maebert.github.io/jrnl/. It uses a single file and allows encryption and basically provides easy simple command line support for adding time-dated entries.