Perfect Combo: Duolingo + Mingaling Plugin
Mingaling is a Firefox word replacement plugin (https://addons.mozilla.org/en-US/firefox/addon/ming-a-ling/) that lets you easily define words to be translated into your choice language, giving you passive exposure to the language you're learning even when you are just surfing Facebook, Reddit, etc...
Until recently, I found the plug-in kinda nice, but I only added words in fits and starts... and I knew it wasn't as useful as it could be.
But today I realized that the killer combo is adding a word from each question in Duolingo, keeping it constantly up to date and in-sync with my language level. And the best thing of all? The way that Duolingo renders its pages keeps the lessons from being translated, so it doesn't mess you up by giving you too much information.
Try it out and see what you think.