An astounding amount of content is available on this app. I personally recommend it for complete beginners. I used Innovative languages initially when I started learning Greek and just watching their videos with dual subtitles, I was able to start reading the Greek script and 3 months later was ordering food while on holiday in Greece. Innovative Language have a dedicated website for each language so click here to redirect to the language menu and get started today!
A fantastic app for those with a little knowledge. italki both works as a kind of social but more importantly allows you to connect with native tutors with whom you can practice your conversation skills in a guided and positive way for as little as $4 an hour!
A great beginner app is Duolingo. It's very intuitive and doesn't mess about. You are thrown into language learning immediately and it's fantastic for complete beginners.
Positives: It's free!
Drawbacks: Some of the languages (Latin/Japanese) are a bit hit and miss. The lack of actual conversation is a real negative within the free content of the app.