Memrise ( http://www.memrise.com/ ) is an online site to assist with memorization of words.
Launched for the public in September 2010, Memrise is at its base a flashcard site. But if that was all, I would have quickly passed over it. The site is for the memorization of word lists, grouped in sections by what language or otherwise they are in; "featured languages" are Mandarin, French, Spanish, German, Italian, and SAT Vocabulary (of course, not a language), and each of these is just a group of many lists. When a user begins learning, it introduces them to their gardening theme: The word seeds, usually in groups of 7, are originally planted in your brain. You grow and water each as you study, and after about 4 hours they are ready to be harvested and placed in your garden to be watered occasionally—you wouldn't, after all, want your pretty words wilting, would you?
The site was founded by Ed Cooke, a so-called Grand Master of Memory, and Greg Detre, with a PhD in neuroscience with a goal to use as much of the science and techniques of memory in a website. The appearance of words is in a "scientifically determined fashion," observable when using it. Every word can have crowd-sourced mnemonics (including images and video), etymologies, synonyms, etc. (though not all do, even on the more popular lists, and yet others have many of each of these things). You are free to review whenever, in several different ways. They get the user to use the spacing effect with the scheduled reviews.
But still, I would have gotten quickly distracted from the site, were it not for the fact that all this combined and with e-mail reminders, should you have created an account that way, make Memrise an incredibly engaging and addictive experience.
Oh, and it's free.