Cardinal is a simple spaced repetition (flash-card) app with Markdown support. It uses a standard algorithm to schedule your flash cards for review based on how easily you recall their answers.
Demo
You can see a live demo of Cardinal at cardinal.mpiroc.com. When I say demo, I mean it. Your data may be lost at any time.
Developers
If you're interested in contributing to Cardinal, there are instructions for developers on the wiki.