There is a similar question/answer where you may find more ideas:
Which Collaboration Software Do You Use?
The tool is one thing, and how you use it is a completely different one. Doing Scrum with distributed teams is more than challenging. Even the CSM trainer said that it was not possible. He suggested to have two teams, separate boards, and a well organized product backlog so that the teams wouldn't block each other. If you believe in this you won't need a tool for a shared board, but you'll need a something for the backlog, which can support concurrent access. We are working like this at the moment, and our backlog is in JIRA.
However, I know a team, where the team members are working remotely and they are using trello to keep in sync. There a small communication overhead for them, which is still cheaper than working together in an office.