Anki (Open-Source: v0.9.9.6) is an app that helps you memorize things.

We have problems remembering things. David Alan made the point that our brain is good at making decisions but terrible at retaining information. But when we do have to memorize something, there are many strategies for doing so. Flash cards are one of the most common approaches, and this is where Anki comes in. It’s program that knows when to show you a card based on how well you remebered it last time and how many times you’ve seen it in the past. There is even a free web-service which can sync your cards with the desktop. I can even access my cards on my iPod. This is not a native mac app, but it won’t look too out of place your desktop. Anki is available to download here for most platforms.