StudyStack is a free site that allows students, or teachers, to create their own flashcards to use as a study aid. You can also search already created flashcards to use. The site also has activities such as matching, crosswords, hangman and more that you can turn your flashcards into to help study a topic. I like this feature because it helps you learn by going through the material in different ways.
As you go through the cards, you can sort them into two piles, correct and incorrect, so that you can review the wrong ones again.
You can access your flashcards on the site, print them out, or export them to a flashcard app. The site supports exporting the flashcards to apps on Android, iOS, Windows Phone, webOS, PalmOS, PocketPC, Blackberry, PC and feature phones. That's an impressive list.
Topics/Categories of existing flashcards include Geography, History, Business, Math, Science, English, Languages, Medical, Test Prep, Aviation, Culinary and much more.
This is another great study resource for students.
StudyBlue - create and share flashcards, quizzes, and study guides