Teaching new technology through app development. The course consists of lessons followed by tasks. Each task takes the app to the next level. Execute tasks in the browser (Codelearn Playground) - no setting up the environment.
With just 2 months of effort, Codelearn stayed at the front page of Hacker News for 9 hours, driving 6k visits & 1k signups - http://news.ycombinator.com/item?id=4213260