Windows only: Remember everything you've learned using spaced repetition with SuperMemo. From the Wired article on its creator, Piotr Wozniak:
SuperMemo is based on the insight that there is an ideal moment to practice what you've learned. Practice too soon and you waste your time. Practice too late and you've forgotten the material and have to relearn it. The right time to practice is just at the moment you're about to forget.... Twenty years ago, Wozniak realised that computers could easily calculate the moment of forgetting if he could discover the right algorithm. SuperMemo is the result of his research.
SuperMemo is available in several incarnations for Windows, Pocket PCs, and Palm Pilots (if you still have one), including a shareware 2004 version and a freeware '98 release. It's not new by any means, so if you've used it in the past, let's hear how it worked for you in the comments.