THIS SITE IS A M A Z I N G! I use it to learn Japanese and I have learnt so many words from it, my vocabulary has greatly improved since using it. This site doesn't only have Japanese it has other languages such as Greek, Spanish, French, German, Chinese etc.
The interface for it is so simple and easy to understand. I love the stars and points system they have as it gives you a sense of achievement and allows you to track your progress. You can also see your rank worldwide, in your country and in your school/institution, which adds a nice competitive element to it. Plus every year they have a giant worldwide competition where everyone can compete to great prizes ( which is pretty cool, you get awarded for learning! What more do you want?! :D )
My only criticism (all those not interested in Japanese on this site have no need to read) is that with the Japanese part it is really really nit picky of spellings of words and sometimes when you get it right it tells you your are wrong. And it won't let you type certain words in kanji that you need to. This gets reeeeally stressful. But you can submit a request to have it corrected (you just click i'm write, 5 second process!) and they will email when they have looked at it and done it for you. But other than that you just ignored that and carry on:)
Another thing I think is really good is that you can use lists that have already been made by other people, students and teachers. Or you create your own list of words you want to learn and make the learning experience more personal to yourself!
Overall I reckon language perfect is the best site to learn vocabulary out there (maybe not so much grammar) and I would highly recommend using it!