It is free to sign up and create unlimited flashcard sets with Quizlet. It is also free to share the sets with anyone by either copying the URL or by embedding the flashcards into your website.
What's new with Quizlet?
Just recently, Quizlet released their own iPod / iPhone App, which also functions on the iPad. Oh, and it's free. Up until this point, you could only view Quizlet flashcards on a mobile devices via 3rd party Apps (that were not always free) or by accessing Quizlet's website. Although this is a great improvement, I still hope there is an iPad specific App in the works.
Overall, Quizlet is one of my favorite online educational tools that students and teachers can use to make learning fun and mobile.