Anki

Anki is a program which makes remembering things easy. Because it’s a lot more efficient than traditional study methods, you can either greatly decrease your time spent studying, or greatly increase the amount you learn.

Anyone who needs to remember things in their daily life can benefit from Anki. Since it is content-agnostic and supports images, audio, videos and scientific markup (via LaTeX), the possibilities are endless.
For example:

Learning a language
Studying for medical and law exams
Memorizing people’s names and faces
Brushing up on geography
Mastering long poems
Even practicing guitar chords!