I've been using Duolingo for over a year to learn Spanish. I love it. I would know hardly any Spanish if not for this app.
Combine it with watching Spanish videos with English subtitles to speed it up.
They did an independent study on the effectiveness of the app too: https://s3.amazonaws.com/duolingo-papers/other/vesselinov-grego.duolingo12.pdf

I'm a fan.