Also known as the Leitner System, spaced repetition is a learning technique that takes memory function into account and places it front and center. We can see it here in Japan used by almost every secondary student with their vocabulary study cards. (photo source)
Digital systems have made these cards outmoded, except in technology challenged situations. With open source software like Anki, digital version of vocabulary cards and any other flashcard content can be input or shared with other users. There is quite an extensive library of flash card sets on all kinds of topics. Anki is usually downloaded, either to your desktop and/or your smart phone (Android/iOS), and it does allow for syncing, even though it is not seamless.
I am curious as to the efficacy of these systems for language teaching and learning. I am considering using Quizlet as a small element in some of my classes, but that does not seem to contain any spaced repetition capabilities. Please comment if you have any suggestions as to alternatives to Anki that are entirely online, and can by used with a web browser.