I notice that Trello is now part of the Atlassian suite (Confluence, Jira, Bamboo). I quite like the Atlassian tools.
I would be curious to know whether people find Slack and Flowdock helpful in the long run. My experience is that you have to come up with some rules of engagement for these tools otherwise they descend into a massive distraction that sucks the productivity out of a team. Certainly on Flowdock you can segregate messages into "groups" which does allow you to temporarily mute the bombardment of messages.