Recently I heard Leo LaPorte in one of the Twit Network podcasts talk about Khan Academy. He was very enthusiastic about it, and recommended it to everyone. I followed his recommendation and checked it out.
Khan Academy is a completely free, donation-driven on-line learning resource. It covers an enormous range of topics, from simple arithmetic, through algebra and trigonometry, up to chemistry, history, and physics. The level of these topics also has a broad range, from kindergarten up to an academic level.
There is a certain game element thrown into the mix by keeping track of energy points. Everything you do - watching a video, doing an exercise - earns you energy points. Apart from the energy points, you can earn badges - ranging from correct answer streaks to becoming proficient in all mathematical exercises.
Khan Academy is definitely worth checking out, especially for people who consider themselves eternal students!