"Variable Passing": How to get numbers or text generated by the problem's Python code into your internal Javascript. Also, how to log your results in the official edX logs.
"Card Sorting": A card-sorting exercise. Used to in cognitive science to measure expertise in a particular field.
"Accessible Matching": A standard matching-type problem as seen in a myriad of classroom quizzes. Designed to be usable on screen readers as well as by sighted users.
"Drop File": A bare-bones problem that uses Javascript to check a file and see whether it's an image or text. Constructed as an example of how someone might create a file-processing problem.
"Adaptive MC": Creates a set of resources where the path depends on a student's answers to MC questions. Still under construction.