Meta Comment: A lot of the comments here that are about getting better at chess seem to boil down to: Do something other than bog-standard chess. Mess with time limits, do openings, play puzzles, etc.

So, can we get together a good list of websites/apps with links that are these 'not-chess' trainers?

I'll start:

https://lichess.org/training

https://store.steampowered.com/app/1349230/5D_Chess_With_Mul...

https://play.google.com/store/apps/details?id=com.FrameOfMin...