I'm taking a class through my university right now that uses LearnSmart from McGraw-Hill. I was pretty irritated having to buy a subscription to a website in order to do my homework ($50 plus the $230 book, seriously?), but they have a study guide that you have to go through to get to the homework that is actually quite helpful. Right up front, it presents you with the question then asks how sure you are that you know it. If you don't know it very well (even if you get it right) it keeps asking the question until its sure you know it. You're supposed to go through this every day (or so), and it will ask different questions mixed in with the older questions.

Worth the $50? That's debatable. Helpful? You bet your sweet ass. When I miss a question then get it right the next time it comes around, it's difficult to describe how I feel.