There is at least one, but I don't know whether it's any good. I'm a bit tempted to write one, partially because I have a thing about self hosting my data, but also because I love the overall idea and design of Trello, but I'd love it to be more extensible/flexible/hackable.

E.g. being able to make layout changes(have lists that span columns for example, or cards that show more of their information on the cover). Or more flexibility in colour changes/background, or ability to style cards.