A free, open-source flashcard program that schedules reviews with spaced repetition.
Anki shows cards just before you are likely to forget them, which helps with memorizing vocabulary, formulas, or exam material. Decks sync across desktop, web, and mobile through the free AnkiWeb service.
The desktop and Android versions are free; the official iOS app is paid and funds development.
An AnkiWeb account is only needed if you want to sync across devices.
Information and links are accurate as of the time of writing and may change. Always verify on the official site. This is an independent guide, not affiliated with the listed services.