I just use a crappy notebook app on my phone. I'll revisit it from time to time, and if there's anything there that's actually useful, I'll make an effort to move it somewhere more permanent. I find ideas have a short shelf life, so I just want something simple.

I have Orgzly set up with Syncthing in my phone for things I actually care about. I don't use emacs, but I like that I get notifications about things on my phone for scheduled things and I have a backup on my computer if my phone dies. I plan to learn emacs orgmode at some point, but I haven't gotten around to it.