Memrise

I've been using the online memorization tool Memrise to review my Chinese. Memrise uses a spaced repetition method, presenting vocabulary items at different times according to how many times the learner has succeeded remembering them, similar in some ways to Duolinguo and the desktop app Anki. They add another element to these systems in the form of "Mems", or customizable visual aids. These can be useful when learning basic vocabulary, though I haven't made so much use of it recently.

Memrise has the advantage of many crowd-sourced lessons, and the ability to create and show your own lesson material. This, of course, means that finding lessons means wading through potentially hundreds of submissions for more popular languages, with varying quality (some of these lessons have some video instruction built in, while others are only simple text). So far, I have spent the most time with a vocabulary set based on the HSK (汉语水平考试 hanyu shuiping kaoshi), but there are a variety of lessons available, and the site lists over 130 languages, including a good number of African and Native American languages, eight sign languages, and even a number of invented languages (including Esperanto, Na'vi and Toki Pona), as well as a host of other topics you can review through the site. Memorization sessions are fun and engaging (if occasionally frustrating when I have trouble with a word).

No tool can replace speaking practice for learning a language, and as said before, the user-made lessons are bound to vary in quality, but all in all I see Memrise being a good place to look if you want some supplemental vocabulary building practice to go along with your overall language learning. And it's currently entirely free, which is always a plus.