We've been using Trello for a week now (coming from Basecamp) and it's pretty great.

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.