Vocabee
Vocabee is a language learning application designed to solve my problem of tracking and effectively reviewing vocabulary from different sources without needless repetition.
In my language learning, at various stages of the journey, I use textbooks, applications like Duolingo, I chat with native speakers or other learners, and analyze texts. I find all of them invaluable, but keeping track of what words are new, or just not very familiar, not to mention practicing in a structured manner, becomes a challenge.
Flashcards are a great way to practice, and the digital form gives them even more power. But because they are such a simple and generally useful concept, none of the implementations I know of are tailored to the specifics of language learning.
Vocabee uses the flashcard idiom for practicing, but takes a more domain-specific approach to tracking and managing vocabulary. It is fit for use (I use it on a daily or near-daily basis), but certainly has some wrinkles and is far from feature-complete.