XXX is a spaced repetition system (SRS) for learning languages. This means that it tracks the words you study, and tries to show you words at the intervals that make you learn as fast as possible—not so long that you forget, but not so short that you waste effort on unnecessary reviews. Spaced repetition is widely considered to be the most efficient way to memorize vocabulary, or anything else.
Spaced repetition was popularized by SuperMemo, and is implemented by many flashcard applications. XXX is distinctive for its:
Flashcards of words in isolation are boring and ineffective. XXX selects from a database of hundreds of thousands of example sentences from a variety of sources, according to the words that you need to review. You won't usually see the same sentence twice.
The spaced repetition algorithm calculates the optimal moment to review each word, but you may not be free to study then. XXX tries to make most efficient use of your time, regardless of how much or little you're able to study in a given day.
If you're already using a different SRS, then you can import a word list (including scheduling), in order to continue your reviews without loss of efficiency. If not, then you can take a placement test. Then add specific words to study, or XXX can suggest the most frequently-used words that you don't already know.
The algorithms that determine how words are spaced and what sentences are chosen are highly customizable. You can evaluate your performance using analytics, and tune the system for the behavior you find most pleasant and effective.
I developed this to help myself learn Chinese. For now, I'm running it as a totally free service, in order to gather data that will help me tune the algorithms to learn more efficiently. In future, I may add paid features or run advertisements. Basic spaced repetition (studying by sentence, repeating by word) will always be free.