a helpful website for duolingo Spanish learners
The website http://spanish.about.com/ for me is the best Spanish support for duolingo learners.
It's teaching single 'problems' we face during duolingo sessions and don't maybe get enough explanations by duolingo itself. E.g. the different use of SER and ESTAR, or the difference between TE QUIERO and TE AMO. It's done with examples from daily life, easy understandable and helpful. Much better than the standard learning books we had in school.
Besides a second application I like (and it's for free on PC and on Android, for I-Phones, I didn't check the availability) is the multi-language dictionary http://enes.dict.cc/ (here for English-Spanish).