What tools for managing Kanban would you recommend?
agilezen.com - simple & effective. The least friction I have seen from any agile planning tool, and as near as I've seen to cards and a whiteboard.
We use a cork board and index cards, much as Pascal Thivent uses a whiteboard and post-it notes. For tracking metrics, we use Excel, although any spreadsheet program would work just as well. This allows us to easily create cumulative flow diagrams.
Update:
The Limited Work In Progress society is now maintaining a list of tools.
You need a tool that will allow you to easily tweak and change your process as you begin to understand your cadence better. You won't get it right the first time, and so you need the flexibility.
Ideally, if your team is colocated, you do this on a whiteboard or cork board as others have said. Low-tech and totally malleable.
AgileZen (aglezen.com) is pretty solid as Jacob mentioned, and new features are being added constantly. I would use this for my team but a hosted solution is not an option for us due to company policies.
Version 4 of the excellent GreenHopper plugin for JIRA has KanBan support. I'm not sure if it's any good, but on the basis of Atlassian's track record, it's almost certainly worth looking at.
If you are using multi-tier boards and like the look and feel of whiteboards and sticky notes, you should check out LeanKit Kanban
Board Image - http://tinypic.com/r/2j3gtjs/6
Presently, LeanKitKanban leads the pack specifically for a Kanban implementation in a hosted app.
All of the available tools are somewhat naive about their interpretation of Kanban and of what's important in a tool and a user experience. They tend to be overly-narrow perspectives of Kanban and Lean by only that part of software product development that is populated by programmers. But then, programmers are the folks most likely to build an app. I hope that we'll get a truly awesome Kanban app/service at some point - one built by a whole product development organization rather than just by passionate programmers jumping on the Next Big Thing.
For the time being, I find LeanKit to be the one that has the least naive implementation of Kanban, but I'll jump at a tool that reflects Lean more holistically and goes beyond mere automated enforcement of work-in-process limits.