I must say that I think that Quizlet.com has won my vote for the best flashcard application on the web. In quizlet, you can create flashcards like any other app. However, the cool things that Quizlet does with your flashcards after you make them is what sets this app apart.
For one, it allows students to play matching games with the words and definitions. It also has a typing race, where students type the words that match the definition. It also gives the audio for many words in 18 languages! Finally, the vocab can be uploaded to mobile platforms like the iPad, iPhone, or Androids!
Check it out!