Anki is a cross-platform app for flashcard-based studying. It uses modern cognitive science to help you learn information faster and retain it for longer.
I maintain a guide that explains how to use Anki, with a brief intro about the science behind it.
The previous link will always point to the latest version of the guide.
Research papers
Sci-Hub provides access to most research papers, even those that are behind paywalls.
Go to its website and click one of its available libraries (they all serve the same content).
Search for the paper you want to access using its DOI e.g. 10.1038/s41586-019-1711-4.
