We find Trello to be effective for our teams. We’ve been using it for over a year without paying any fees and we’re managing several projects or boards simultaneously. We’ve integrated it into Slack, which is a great internal project communication tool that’s free. All changes or additions in Trello notifies our slack group that’s affiliated with the project. There are some features that require a premium account, but you can work for the most part on the free service without harsh limits.
Rey’s original response can be found here on Quora