In the past I've written about Babbel for mobile and Rosetta Course (mobile version of Rosetta Stone) for language learning when you have spare time. Now another contender enters the ring, and has a lot of promise.
Duolingo turns learning a language into a game, more so than the other apps. You are given a few hearts (representing your health pool) that you don't want to lose during the learning levels. Hearts are lost by answering questions incorrectly. Once you finish a lesson with hearts remaining, you'll earn an achievement to track your progress in the app.
To get started, you'll just need a free copy of Duolingo for your Android device. You can sign-up for an account or use Google+ or Facebook logins. Make sure to check out the privacy settings on both of these social media options before you tap OK. I took away the app's ability to post to my circles or even see some of them on Google+.
Currently Duolingo's language choices are relatively limited, but still sufficient if you're trying to learn one of the more popular ones (though Chinese is missing). You can choose from Spanish, German, French, Portuguese, or Italian. Duolingo also offers English translated to all of these languages, except German, for learning in the opposite direction.
The lessons are very similar to Rosetta Stone methods -- meaning that you're instructed to pick the picture that matches the words at the top of the screen. Duolingo requires a bit more interaction when checking your answers than the other two apps mentioned above. Though, as a bonus, this app offers some more practical exercises for learning the words, their spelling, and associated grammar. There's even times when you'll need to type in the words you're learning. Perhaps this is the reason Chinese isn't supported yet?
Overall, you get an amazing set of learning tools for free. Whether the methods used for teaching are better than another app's methods is probably dependent on the student. What do you think? Better or worse than other options? Share your thoughts in the comments.