Hindsight is a free and open-source tool for executing Kanban-based sprint retrospectives. It's private (no back-end, data is shared between connected browsers), clean and easy to use and has built-in timer and music player.
The application is easy to use and work in a group, it has a music player that makes the experience much better and it's free. My team has been using it for scrum retrospectives and it's been great.
