Ask HN: Which tool for lean Kanban?

Which tool or Trello add-on do you use to manage a Kanban board?

We currently use plain Trello with a Kanban-ish process. Trello is great for card creation and update, but sorely lacking in reporting (e.g. how long does it take for a card to traverse the board) and advanced options (e.g. limit the number of cards per column or total WIP, create a fast-track slot in each column, etc.). Which tool or Trello add-on would you recommend that combines Trello's ease of use and advanced Kanban features?