I am currently using Duolingo to learn Spanish and I find it quite amazing. It offers a great interface and I love the gaming elements. It's by far one of the best web applications I have used in a long time. So kudos to Luis von Ahn and the team!

I recommend watching his TedX talk (the mind boggling thing from the presentation is: English Wikipedia could be translated in 80 hours if 1 million people used Duolingo!) http://www.youtube.com/watch?v=cQl6jUjFjp4