I also recommend Pimsleur. Unlike with Duolingo, I feel like I can actually produce new sentences with the constructions that I've practiced with Pimsleur. Of course, Duolingo covers more vocabulary, but with Pimsleur I feel like I can actually speak the words and produce new sentences, not just type/recognise them.

(I've also heard Assimil is a good alternative to Pimsleur.)