Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Elegantt for Trello - The most elegant way to manage your Trello projects
Planyway - Visual project management for Jira and Trello: manage resources, build project roadmaps, use time tracking.
Hello Epics for Trello - Simplify your Trello board with parent/child relationships
Corrello - Dashboards for agile teams using Trello
TunnelVision - See your next Trello tasks every time you open a browser tab