I was using www.flashcardexchange.com for a while. I still think this is a great site. It’s easy and there are a lot of people who have made advanced-level Korean vocabulary cards (or any language for that matter).
But, I just discovered www.quizlet.com and I think it is great. It’s in the stages of developing, so maybe it will get even better. For now the main advantage is that it formulates mini-quizzes that you can take. You can also use the app for the iPhone.