Trello looks pretty good, will have to have a deeper look.
Realtime updates are deceptively complicated to get right. I bumped that whole branch of development so I could get something out (really try to work some of the ideas of Lean and the Minimal Viable Product).
We really like the following features:
- It's free
- It's Realtime, every action (drag-drop edit etc) get's pushed to the other users.
- Great UX because it's made by the guys (or some of them) that did Stack overflow.
- Responsive design so we can use it on our webkit enabled phones.