I literally downloaded every single free app that came up under "Spanish" in the iTunes store. Duolingo was on my backlist, because it required registration immediately before you could use the app. Those apps I usually delete straight away. But since you say it's good, I went ahead and registered. Didn't get too far, since very quickly it required headphones to complete a task, and those are at home (it's impossible to hear anything on the subway).
So far I've been using: iSabi (a vocabulary list but each work also has two fairly complex sentences that use the word in context, all of which you can listen to), busuu (phrases and then a series of tests - multiple choice, complete a sentence, reading comprehension and writing - for each set of phrases), IntroSpanish by Brainscape (flashcards of words and phrases, and you rate 1-5 how well you knew that, to determine how soon the app shows you that word again - I like that quite a bit).