I like whiteboards.
Corkboards are nearly as good, but harder to reflect changes in process on, IMO. While pinned index cards don't fall off as often as stickies, if your stickies are falling off you might want to look at how long it's taking to get them off the board in the first place.
I've also seen cupboard doors, table tops and windows suborned into the role. These also work well.
If you have no board, no wallspace, no cupboards, table tops or windows then I guess you could use LeanKitKanban or AgileZen. They're free for 1 user and pretty cheap for large teams. I don't know of any open-source solutions.
Otherwise... whiteboard is the way forward.