I have started using the Memrise app for a few months now and I find it quite good and effective. However, in reading Fluent Forever, it recommended using the spaced repetition or SRS method.
And the very much talked about SRS app is Anki.
The PC app is free and apparently it is a very powerful tool. But its iOS app cost $25! That is crazy.
But there are many who swears by it and felt that it is worth the $25.
It was said that the developer spent years developing the app supported by donations but it wasn't enough to sustain his work and so when he created the iOS app, and charged good money for it. So in a sense, it's like a donation to support his work across all the platforms. I suppose it sort of make sense in that he could also distribute the charges across all platforms, meaning I would have to pay for the PC version I am using now for free.
It is frustrating though that the Android app is free of charge. It seems however, the Android version was not developed by him.