Iknow - is a collection of flashcards authored by excellent teachers in different fields. 

Iknow has a review algorithm based on leitner system that helps you memorize all you need.

Besides many decks in high school courses and the Quran, Iknow includes:

Barron's Essential words for the GRE (the 800 GRE words)

Magoosh GRE VOCAB FLASHCARDS