Sometime last year (actually, the year before now), I discovered Lingt, a beta site that was using online flashcards in new ways. It was mostly for learning Chinese, and students could create their own material, or use pre-formatted sets of words and phrases.
The site uses a spaced repetition method to ensure that you re-see the words you need to review at the ideal times. It also tests (with English, pinyin and simplified characters) multiple ways of ‘knowing’ a word – recognition, recall, listening, writing. It even does it all in a cute, clean interface.
Sadly, I just got notification that the site will be closing down at the end of February this year. I didn’t use it regularly, but it was a very useful tool. On the upside, their technology has been bought by Dictionary.com, who will be using it for their own flashcards section. Hopefully this means that it will be brought to a bigger audience in more languages, and that can only be a good thing.