Memrise - Grow your mind
I would like to introduce you to Memrise.
This is app (IOS and Android), as well as a website that will boost your vocabulary in the language (or topic) that you want to learn.
Memrise will not only teach you new vocabulary, but also relentlessly present you again and again with those words, until, well, you know them. The clever bit I think is that there is always a delay between the moment you learn a new word, and when you are tested on it. To begin with, it is counted in hours, and as you progress it changes to days, or even weeks or months.
Memrise is trying to mimic the process by which we memorise new words. And to some extent it works pretty well. I managed to learn about 1500 new words in Russian in very little time; considering that the lack of vocabulary is certainly the biggest problem to language learners, I would say that Memrise fills in that niche nicely.
One great feature for my students is that you can download the whole vocab set on your device to use it offline later (like on a bus or on a plane). There are thousands of contributors to the Memrise courses, so I am sure that you will find what you need. For instance, someone kindly posted all the vocabulary for our textbooks (in their relevant units!), and this makes an excellent learning tool outside lessons for my students.
Another great feature is that some courses come with some recorded audio too (that is downloaded with the course); I know that this is an invaluable feature for me and my Russian words. But the best feature is that it as absolutely free, so you should really try it for yourself!