Duolingo for Schools is now open to everyone. I planned to write a longer review of it, but Larry Ferlazzo wrote one earlier that is better than what I had to say. Larry is using the service with his students so I encourage you to read his review.
Duolingo is a platform that offers activities for learning Spanish, English, French, Italian, Irish, Dutch, Danish, German, and Portuguese. The service works in your web browser and is available as an app on Android, iPad, and Windows 8. Duolingo for Schools allows teachers to assign activities and monitor students' progress.