Anki is a free and open source piece of software that lets you create flashcards and schedules when you should review them.
I have a few decks which I've shared on AnkiWeb. Notably:
(Sharing through AnkiWeb means that users can more easily receive any updates to the decks.)
I also have some tips for using Anki for mathematical content which you can find here.
All my decks are licensed under the Attribution-ShareAlike 4.0 International license. You have permission to copy or share it, as well as adapt it for any purpose (including commercial) – provided you give adequate attribution and license your changes under the same permissions.