WordStash is another option in the Web 2.0 software that allows users to create Flashcard based study guides and then use the list to study, play games, and share words with friends and students. WordStash allows users to also add images to their flashcards, or create flashcards based on images. This could be useful for special education students, foreign language classes, and especially science and social studies.
One feature noted on the front page is that WordStash uses Science Based Learning to calculate the most efficient time to study using a spaced repetition algorithm. The software can also save words you look up in the dictionary.
You can create an account for free or view list already created by other users. One of the games is waterfall, where you have to type in the correct answer before the definition or image disappears at the bottom of the screen.
Here is a short tutorial that someone created that might be worth checking out.