The Best Free Collaboration Software
We find Trello to be our favorite collaboration software. We’ve been using it for over a year without paying any fees and managing several projects or boards simultaneously. We’ve integrated it into Slack, a free internal project communication tool. Trello notifies our Slack group of any changes to the board. Some features require a premium account, but you can work for the most part on the free service without harsh limits.
Furthermore, Rey’s original response can be found here on Quora