Trello. Its flexible, powerful, and easy to integrate with other apps you use via Zapier and more (disclosure: I work on the Zapier team). I use that with OmniF

Trello. Its flexible, powerful, and easy to integrate with other apps you use via Zapier and more (disclosure: I work on the Zapier team). I use that with OmniFocus' email drop to keep my todo list and due Trello cards synced, for instance.