As mentioned above, Anki and Supermemo are two free implementations. As for ready-made flashcards, here are some I made for a calculus class:

http://www.spicyelephant.com/decks/523 http://www.spicyelephant.com/decks/577 http://www.spicyelephant.com/decks/659

You probably have to register for an account if you want the site to apply supermemo-type techniques. I don't recommend using the site in the long term because it will eventually force you to register for $20.