In a nutshell: Smart.fm is a free, web-based flash card service. If you crack the nutshell open to get a look at the details of how it works, though, it’s far better than that. Here, let the official Smart.fm video introduction explain what I mean.
Note: They have rebranded altogether since this was originally posted on my old blog. The video was taken down and the service has a new name. They don’t seem to have a video anymore, but their features page can be found here.