Started really simple, but I spent a little extra time to use the Ace Editor, Web Workers, and give it a responsive design.
List Tool - http://skratchdot.github.com/list-tool/
Source Code - https://github.com/skratchdot/list-tool/
List Tool - http://skratchdot.github.com/list-tool/
Source Code - https://github.com/skratchdot/list-tool/