It's built by a team of interns at Rally. We'd love to know why you'd use it for private repos but not open source projects, hoping to help people know where they can start contributing to an open source project.
Keeping straight what stage the Issues in our private repos are in, though - that's a very real problem I deal with every day. I'd love to be able to surface our internal company flow in a simple way like this; it's very kanban-like, and doesn't require us to add another tool or try to keep things in sync since it sits right on top of Github Issues which we already use.