You could try Memrise or Duolingo, of which the first has large numbers of user made stacks, but which I usually find have enough errors or other problems as to become rather irritating to work with. Duolingo is awesome, so long as what you want to learn is on their fairly small list of languages, as it has some understanding of grammatical issues, and far fewer outright errors than Memrise. My main issue with Duolingo is that its courses are somewhat limited - even if you finish one, you won't have much more than very basic literacy and vocabulary in the language.

Personally, now, I am concentrating on Anki, and I build my stacks by adding words and short phrases as I come across them during the day. It is an excellent, if clunky, tool: it allows you to add TeX for learning equations, etc; its model lets you define different types of data structure ('Note'), from which you can define different question-responses to learn ('Cards'). The main problem is the limitations of the iOS app, which doesn't let you add new Notes or process your TeX for you.