I built http://iijo.org. Its free and does Chinese flashcards and uses spaced repetition. You can build vocab lists from a built in dictionary. You can also directly search the dictionary: http://iijo.org/dictionary.
I found that duolingo is actually not that good for advanced students - everything I saw on there seemed to be focused on learning by translating phrases into English. Thats pretty distracting once you get past a certain level.
I've used duolingo for Spanish review; it's pretty good. Honestly the best thing about it is the UX: you get little bite-sized exercises you can go thru as fast or slow as you want, review, etc., so it's easy to squeeze into a 10-20min/day block. The main weaknesses are a) the examples are arithmetically generated so sometimes you get "My horse only eats milk" or something as an example sentence (it's usually pretty good tho), and b) you don't get a deep grammar review, it's just lots of little examples & practices. The latter point could be seen as a "pro" depending on your learning style.
I've used Transparent recently and would recommend it. Not free but a lot cheaper than Rosetta Stone at $29 per month. They have a free offering called Byki as well which I haven't tried.
Although not specific for languages, memrise.com has flashcards for a handfull of languages. It also remembers what you got wrong and reiterates accordingly.
For discussion and ideas: https://news.ycombinator.com/item?id=5485073