Vocage is a minimalistic terminal-based vocabulary-learning tool. It presents flashcards using a spaced-repetition algorithm (e.g. Leitner); words you know well will be presented less and less frequently and words you have problems with will be repeated more often. You quickly move cards/words between decks and each deck has an associated presentation interval.
You can use vocage for anything you'd use flashcards for and not necessarily limited to learning languages.
The aim of this software is to keep things simple and minimalistic and to focus on one thing only (the unix philosphy).