This one called Flashcard Exchange already has some for the Small Catechism thanks to many who found it before me. You can sign up for free and create your own for Scripture memory work. It also has a quizzing utility. The cool thing on this site is that there are cards already created for many Catechism categories including the questions and answers. It would be cool to use those questions with the Bible reference answers. This is a cool site but there are so many ads that it feels really complicated to use. Mobile apps are also available.
There's also one that I consider a step up (and I'm not sure why) called StudyBlue. The quizzes are pretty good and it keeps track of scores. There isn't anything for the Catechism or confirmation already created but when I looked at religion there were a few created about Martin Luther and/or Lutheran History. It also has apps for Apple and Android phones so kids can take their memory work with them wherever they go.
And then there is Quizlet. For some reason I liked this one best and it seems like it's well used. It also not only has flashcards but keeps track of what is learned, creates quizzes with short answer, matching, and multiple choice questions and also has some already created. As with the others the user can determine what types of quiz questions they want and whether or not they want to be prompted with the word/phrase or the definition. It is also available for Apple and Android phones.
CHECK THESE OUT!