Earlier quoted context omitted.
it's not a simple idea, it's a deep idea refined over decades and implemented simply. http://en.m.wikipedia.org/wiki/Kanban
I work at Trello. We actually don't think of Trello as Kanban. Trello at its core is a collaborative list of lists. You can use Trello to implement a Kanban workflow but Trello is not an implementation of Kanban. A lot of people use Trello for things like to organizing their soccer teams where there isn't a hint of Kanban -- each list could be a game and people add themselves to indicate that they want to play.
Some people use it for completely non-tech things, a couple I know use it to track their sales pipeline.
It's pretty amazing that there really wasn't something quite as simple made before it, it seems so obvious in retrospect. The implementation is spot on.