Take a look at these simple pm tools:
Take a look at http://casual.pm/ it helps visualize and manage flow of tasks/actions (including gamedevs workflows).
I don't know any such tool (beside Casual) that allows to design and manage project workflows.
Take a look at these:
https://trello.com/ - shared task boards (powerfull but simple and free tool) http://casual.pm/ - visual project management tool (helps to visualize and manage project workflows).