It uses the most popular Stack Overflow questions, and sprinkles it with some spaced recall algorithms. The goal of this spaced recall is to help the student move answers into long-term memory. Stack Overflow is a natural good source of continuous, peer reviewed, modern Q&A content. (Heavily inspired by anki, an excellent flash card program.)
Code licensed under the MIT license
Ruby on Rails, PostgreSQL & Redis
https://github.com/stefan-lz/topstack.io, contributions welcome.
By leaving a comment below.