I also used those things! I used Anki and Quizlet (which isn't open source but you can import your own word lists so it doesn't feel too "sticky"). Duolingo was

I also used those things! I used Anki and Quizlet (which isn't open source but you can import your own word lists so it doesn't feel too "sticky"). Duolingo was still key, though - it felt less like work, because it was nicely gamified. (I give all this praise with a twinge of dread now, because I'm sure that with the IPO, Duolingo will be twisted in the way that all publicly-traded companies are).