Officially Recommended Resources?
External resources and materials have been a vital part of my learning experience on Duolingo. What's remarkable, however, is that all of them are peer-recommended, collected and explored as I progress in the skill tree and flip through pages upon pages of comments and discussions. So far I've been able to learn from (great thanks to my contributors):
german.speak7.com, german.about.com, www.omniglot.com on general knowledge, grammar, and trivia; forvo.com/languages/de/ for pronunciations; dict.cc, www.duden.de for definitions and pronunciations, the latter being a German-German dictionary
Despite this arsenal of resources, I'm still well aware of the amount of information NOT provided on Duolingo. What else explains that "Heck, I never knew that" feeling, especially when it comes to grammar?
Not only do I hope that even better stuff will be shared on this great website in the future, I am also calling for an official evaluation and recommendation of the wide spectrum of resources available online as well as offline, virtual as well as physical. We understand that Duo is not everything there is to German or whatever language, and we crave that knowledge, so that more may be learned and shared.
Up-vote this post if you agree with me to get the developers' notice, and please feel free to share and discuss whatever you've found useful to your learning. CHEERS! :)
I've found Deutsche Welle's Learn German section really helpful (http://www.dw.de/learn-german/s-2469). There's free online lessons that start from the beginning and go up to level B1. There's even quizzes and tests along the way. I also recommend their audio lessons called "Deutsch - Warum nicht?" They've helped a lot with listening comprehension and the story the lessons tell is genuinely interesting (in my opinion, at least). For higher levels, they have videos with exercises. Honestly, I owe a lot of what I know to Deutsche Welle's resources (along with Duolingo, of course).
I know a very good grammar, but it's in Italian, http://www.tedescoinrete.it/.
My favourite site to check all the declinations and conjugations is http://canoo.net/
My favourite dictionary (German only, but with many explanations, examples, links) is http://www.dwds.de/
And people from Deutsche Welle are amazing. They welcome feedback and there are many different contents for different levels.
I would always recommend www.leo.org even though some dislike it for its user generated content. This is the place I get most inspiration when searichng for a good translation. Often I use it more like a thesaurus as it gives me a good overview of the different connotations and denotations.