Memrise is an awesome free learning app which makes use of spaced repetition to help you learn languages and study for other subjects.
I’ve only ever used the language side of things, which I’ve found to be great. You can download pre-made courses for beginners, intermediates or advanced level in tons of languages. You can also download user-made courses for specific vocab (e.g. the 1000 most used works in a language) or create your own courses.
A while ago I put together a language learning quick start guide — it has a step-by-step routine for getting started on Memrise.
If you want to learn more about one of Memrise’s co-founders, Ed Cooke, check out Moonwalking with Einstein by Josh Foer.
You can find out more about memory techniques and impressive feats of memory on the Find A Spark memory homepage.