ProVoc has a great quiz and card editing interface. It also allows a 3rd side to its cards (which is great for Chinese because you need the English, Chinese character, and the pronunciation). It has very useful features BUT and this is a big but, the study algorithm does not work. Supposedly the program can quiz you based on difficulty and the last time studies. But in my month long experience with this program it did not seem to work. I kept getting quizzed on the same words over and over instead of introducing the words that hadn't been studied in a while or have a high difficulty. And this even after I moved the difficulty slider during the quiz.

If you're looking for a good study algorithm and a free program, try Genius. It works much better for me even though it only allows two sides to your flashcards.

ProVoc would be the program of choice if not for the algorithm issue.