I have found Duolingo much, much less useful for language learning than Language Transfer. The latter actually helps you learn to think in another language rather than memorize things (which is still useful, but not nearly as much).
Short if total immersion, I have found nothing better than LT.
The problem I have with finding an alternative is that most just offer some five to ten largest languages. Want to learn Spanish, French, Russian, or Chinese? There are hundreds of both free and paid services available. Want to learn Hungarian, Irish, or Finnish? It’s Duolingo and a scant handful of sites specific to that language.
Just audio. But it is presented in a way that helps you to learn, rather than just remember. If you give it a try, I promise that you will be shocked at how you can retain the knowledge.
It isn't enough on its own, however. You need to reinforce the lessons by speaking to people, reading, and/or TV and movies.