Lingvist.io has an interesting foundation story, where (in short) Mait Müntel, one of its co-founders, had to develop a solution of his own for learning French. The meat and potatoes of Lingvist is “Memorize”, a Spaced Repetition based system where you learn words in context, and get them read back to you once you get the answer right. This obviously means that at first you’re going to have (almost) no idea of what’s being asked of you, but this approach is similar to how we learn new words in the real world, so it’s actually a very good idea. It also has a Reading and Listening section with an amazingly large collection of texts and audio (with scripts) that you can use to reaffirm what you’ve learned in the Memorize section. Currently, you can learn French and English from Estonian, English, French and Russian. They’ve also released apps for both iOS and Android.
I used during my 2015 French language mission, and out of all the resources I used this gave me the highest vocabulary retention rate, without a doubt. I highly recommend it, even while it is in beta phase, but it is worth mentioning that once the beta is over, Lingvist.io may stop being free, although no price chart has been mentioned yet.