Anki is described as an "friendly, intelligent flashcard program." You can find details information and instructions on how to use it.
I, my wife, and many friends have been using it for over two years to study Japanese and have found it to be extremely effective. There is an addictive element to Anki that makes us want to study more and more - a wonderful thing when trying to study anything.
Also, the iPhone app makes it extremely easy and effective to study while walking, on the train, and in those few minutes between other things. By using it in little bursts throughout the day, a lot of studying can be accomplished.