Fantastic! And very useful just the way it is. One way to take this further might be to enable the user to click "correct" or "incorrect" and get a retest at the end of those words he/she failed to get. This wouldn't have to be remembered in the system forever -- even a local one time test would be quite useful reinforcement.
Again, great stuff. By the way, for any users not currently donating to the site, please do! This site is worth thousands to each of us.