One of the most popular education apps on the market, Duolingo offers free language-learning by way of gamified lesson plans. The app includes programs for English speakers to learn Spanish, German, French, Portuguese, and Italian, as well as programs to learn English from any of those languages except German.
When you first boot up the app, you must either create a new account or log in with existing Facebook or Google+ credentials. With your progress tied to an account, you can easily continue your lessons on different devices without having to start over. The convenience here is clear, but I would even suggest taking it a step further. I'd love to see a quick account switcher built into Duolingo. As it is now, the app makes you log out and log back in to switch, which is less convenient for couples or even families who share a device.
Each language program is split into different levels, with Basics being the first, naturally. Levels are then split into different lessons, which focus on specific skills and vocabulary sets and usually take 10 or fewer minutes to complete. At the end of each level, there's a practice test that you must complete before you can move on.