anki

Anki is a program designed to help you remember facts (such as words and
phrases in a foreign language) as easily, quickly and efficiently as possible.
To do this, it tracks how well you remember each fact, and uses that
information to optimally schedule review times. With a minimal amount of
effort, you can greatly increase the amount of material you remember, making
study more productive, and more fun.

While Anki can be used for studying anything, it also ships with special
features designed to make studying Japanese and English easier: integrated
dictionary lookups, missing kanji reports, and more. Sample decks are also
provided for Russian.